USAGE
    ni [commands...]              Run a data pipeline
    ni --explain [commands...]    Explain a data pipeline
    ni --inspect                  Interactive documentation and literate source
    ni --js                       Interactive 3D visualization
    ni --upgrade                  Upgrade to latest version
    ni --version

    This documentation is not exhaustive; see 'ni --inspect' for everything.

    ni outputs progress while it's running; see ni //help/monitor for details,
    or export NI_MONITOR=no to disable.

    ADVANCED
    ni --upgrade develop          Specify branch for upgrade (default is master)
                                  Note that this may downgrade ni; this allows
                                  you to use --upgrade to switch branches.


SYNTAX (ni //help/stream)
    ni chains commands (operators) with shell pipes, which means these two
    commands are equivalent:

    $ ni //help r5
    $ ni //help | ni r5

    Operators that write data usually append to existing streams:

    $ ni //help //help
    $ ni n10 //help

    You can omit whitespace and brackets unless the omission makes the pipeline
    ambiguous:

    $ ni //help FW g c O r5
    $ ni //help FWgcOr5

    Numbers can be written several ways:

    $ ni n100
    $ ni nE2
    $ ni n='10 * 10'


DOCUMENTATION (ni //help)
    //help/ni_by_example_1  or  //help/ex1
    //help/ni_by_example_2  or  //help/ex2
    ...
    //help/ni_by_example_6  or  //help/ex6
    //help/ni_fu
    //help/cookbook

    ni --inspect            Webserver with interactive docs and source explorer

    ADVANCED
    //ni/keys r/^doc/       All documentation pages
    //ni/doc/net.md         Open a documentation page by addressing ni's state
    //help/net              Shorthand to open doc/net.md

    //ni                    Output ni's source code
    //ni/keys               Output ni's internal data state

    $ ni //ni/keys r/^doc/  List all builtin documentation pages

    Note that //ni and //ni/keys will differ if you bind data closures or
    runtime libraries.

    --explain [commands...]      Explain a data pipeline before meta-expansion
    --explain-meta [commands...] Explain a data pipeline after meta-expansion


GENERATING DATA (ni //help/stream)
    filename        Write the contents of a file, decompressing if necessary
    dirname         Write a list of directory contents
    http[s]://url   Write HTTP GET stream using curl
    file:///path    Write contents of a file, decompressing if necessary
    i'text'         Write 'text' as output
    i[x y z]        Write 'x y z' as a tab-delimited output line
    k'text'         Write 'text' forever
    k[x y z]        Write 'x y z' as tab-delimited output forever

    n100            Write 1..100 to output, each on its own line
    n='3 + 4 * 5'   Write 1..23
    n0500           Write 0..499
    n               Write 1.. as an infinite list of integers
    n0              Write 0.. as an infinite list of integers
    n%7             Write 0..6,0..6,... as an infinite list of integers

    _               Vertically align line contents within 1024-row groups

    It's common to end a stream with '_', which vertically aligns multi-column
    data.

    ADVANCED
    fileseek://64:foo   Contents of file 'foo' beginning at byte 64
    filepart://5:2:foo  Two bytes of file 'foo' starting at byte 5
    zip://file.zip      List contents of zip archive (each is a ni URL)
    tar://file.tar      List contents of tar archive (also handles tgz etc)
    7z://file.7z        List contents of 7z archive
    git://dir           List git sub-URLs for git-managed dir

    dir:///path         List URI-form filenames in a path, unsorted
    ls:///path          List plain-text filenames in a path, unsorted (fastest)

    s3u://bucket/path   Contents of 'aws s3 cp s3://bucket/path -', unsigned
    s3://bucket/path    Contents of 'aws s3 cp s3://bucket/path -', signed
    s3r://bucket/path   Same, but with --request-payer (requires NI_DANGER_MODE)

    s3lsu://path        'aws s3 ls --recursive --no-sign-request'
    s3ls://path         'aws s3 ls --recursive'
    s3lsr://path        'aws s3 ls --recursive --request-payer' (NI_DANGER_MODE)

    sqlite://file.db    List tables in database (each is a ni URL)

    wiki://JPEG         Read English-wikipedia JPEG article as source
    enws://JPEG         Read EN Wikipedia article as Source
    simplews://JPEG     Read Simple Wikipedia article as Source
    zhws://北京市       Read ZH-language article on Beijing


COLUMNS AND FIELDS (ni //help/col)
    fA  or  f       Select first TSV column (columns are A..Z)
    fBA             Swap first two columns, discard others
    fBA.            Swap first two columns, followed by everything else
    fAA             Duplicate first column, discard others
    fA-D            Select first four columns
    f^D             Copy fourth column to front (== ni fDABCD.)
    x               Swap first two columns, keep others (== ni fBA.)
    xE              Swap first and fifth columns (== ni fEBCDA.)

    Columns can also be specified numerically: f#0,#1,#2 == fABC.

    F:/             Use '/' as a field delimiter; i.e. split on '/'
    F/foo/          Split on text matched by /foo/
    Fm/foo/         Scan each row for /foo/, emitting each as a field
    FC              Split on commas (== ni F:,)
    FD              Split on '/' (== ni F:/)
    FV              Split CSV, except fields that contain newlines
    FS              Split on horizontal whitespace
    FW              Split on non-word characters
    FP              Split on pipe symbols

    F^S             Join fields with space characters (inverts FS)
    F^C             Join fields with commas
    F^:%            Join fields with '%'

    ADVANCED
    w[...]          Zip each line with a line from 'ni ...', joined on the right
    W[...]          Zip each line with a line from 'ni ...', joined on the left

    W[n]       or  Wn       Common idiom: prepend line numbers
    W[kfoo]    or  Wkfoo    Prepend each line with 'foo'
    w[k[x y]]  or  wk[x y]  Append 'x y' to each line (as TSV)


ROWS (ni //help/row)
    r10             Take first 10 rows: head -n10
    rs10            Print first 10 rows but consume all input
    r+10            Take last 10 rows: tail -n10
    r-10            Drop first 10 rows
    rx10            Take every 10th row, evenly spaced
    r.1             Take 10% of rows, sampled randomly but deterministically
    r/foo/          Take rows matching the Perl regex /foo/
    r'/foo b*/'     Take rows matching /foo b*/
    rp'length > 5'  Take rows for which the Perl expression 'length > 5' is true
                    (see ni //help/perl)
    rA              Take rows for which column A is non-blank
    rpa             Take rows for which column A is non-blank and nonzero

    R^              Collapse lines by replacing \n with \r
    R^4K            Collapse lines with \r until they're at least 4KiB long
    R,              Fold lines by replacing \n with \t
    R,128           Fold lines with \t until they're at least 128 bytes long
    R=foo           Replace 'foo' with \n within collapsed stream
    R/foo.*?bar/    Emit each match of /foo.*?bar/ on its own line

    ADVANCED
    JA[...]         Outer join unsorted stream with 'ni ...' on column A value
    jB[...]         Outer join sorted stream with 'ni ...' on columns A and B

    riA[...]        Take rows whose A field is included in 'ni ...'
    rIB[...]        Take rows whose B field is excluded from 'ni ...'
    rbC[...]        Take rows whose C field is in the bloom filter generated by
                    'ni ...' (see zB operator to create bloom filters)
    r^bC[...]       Take rows whose C field is _not_ in the bloom filter
                    generated by 'ni ...'

    ry'...'         Take rows for which the Python expression '...' is true
                    (see ni //help/python)

    rm'...'         Take rows for which the Ruby expression '...' is true
                    (see ni //help/ruby)

    rl'...'         Take rows for which the Lisp expression '...' is true
                    (see ni //help/lisp)

    rjs'...'        Take rows for which the NodeJS expression '...' is true


CELLS (ni //help/cell)
    ,Cd             Clean cells in column A as integer (remove [^-0-9])
    ,CfD            Clean cells in column D as float
    ,Cw             Clean non-word characters
    ,Cx             Clean non-hex characters

    ,s              Calculate running sum of cells in column A
    ,sAC            Calculate running sums of columns A and C
    ,d              Calculate delta of first column
    ,0              Column -= first value (offset to set first value to zero)
    ,a              Calculate running average of first column
    ,aw5            Calculate running 5-windowed average of first column
    ,q              Quantize cell values (round to nearest integer)
    ,qAB.05         Quantize cells in columns A and B to nearest 0.05
    ,l              Log-transform values in column A
    ,lC2            Calculate base-2 log of values in column C
    ,e              Exp-transform values in column A
    ,eB2            Calculate 2^x for values in column B
    ,L              Log transformation for signed data

    ,agA            Group rows with the same A-column, then calculate the
                    average B-value for each group (output is A, mean(B))

    ,sgC            Group rows with the same A, B, and C columns, then calculate
                    the sum of D-values for each group
                    (output is A, B, C, sum(D))

    ,qgB4           Calculate quantiles for C values within each (A,B) group;
                    "4" means you'll have min, 25%, 50%, 75%, max -- i.e.
                    quartiles with bounds

    ,z              Dictionary-compress each distinct cell value to an integer
    ,Z              Count changes in the cell value
    ,h              Murmurhash each cell value
    ,H              Murmurhash each cell value, adjusted to unit interval
    ,m              MD5 each cell value

    ,t              Convert UNIX epochs to ISO-formatted timestamps
    ,g              Encode geohashes from "lat,lng" strings
    ,g5             Encode geohashes at precision 5
    ,G              Decode geohashes into "lat,lng" strings

    Consecutive cell operators can share the initial comma: ,ls == ,l,s


IO AND COMPRESSION (ni //help/stream)
    \>foo           Write stream to file, then output the filename when done
    \>              Identical to \>, but generates a tempfile name for you
    \<              Read stream from filename (inverts \>)
    :foo            Save data checkpoint in file 'foo', reusing it if it exists

    z4              Compress with LZ4 (with lz4)
    zo              Compress with LZO (with lzop)
    z  or  zg       Compress as gzip
    zb              Compress with bzip2
    zx              Compress with xz
    zz              Compress with zstd
    zn              Redirect to /dev/null (lossy compression)

    zd              Decompress stream contents, autodetecting type (includes
                    bare zlib/deflate streams)

    z9              Compress with gzip -9
    zz19            Compress with zstd -19
    z42             Compress with lz4 -2

    ADVANCED
    zB42            Produce a bloom filter for 10000 items with 0.01 FP rate

    \|              Lazily write stream to FIFO, output fifo name immediately
    \<#             Like \<, but unlink after reading (requires NI_DANGER_MODE)

    W\<             Like \<, but prepend the input filename to each line of data
    W\>             Write each line to filename in column A (column-A values
                    should be contiguous)
    W\>[...]        Write each line to filename in column A, preprocessing each
                    stream with 'ni ...' (column-A values should be contiguous)
    S\>[...]        Like W\>, but keeps files open so column A doesn't have to
                    be contiguous (if you have too many distinct values, this
                    will fail with "too many open files")
    \*[...]         Like S\>, but keep outputs instead of writing files -- note
                    that lines are not merged (for structured merging, use the
                    S[...] operator in //help/scale)


SORTING AND COUNTING (ni //help/row)
    g               Sort the whole stream using constant memory
    o               Like 'g', but numeric sort
    O               Descending numeric sort
    c               Count and merge adjacent identical rows (uniq -c)
    u               uniq
    U               uniq -c across all rows, done in memory (output rows are
                    randomly shuffled)
    wcl             Shorthand for wc -l

    gBA             Sort using columns B and A to determine order
    gBnA-           Sort using B numeric, A reverse lexical to determine order
    gCg             Sort on C numerically, parsing scientific notation (not all
                    'sort' programs support this)

    ggAB            Group rows with the same A-column value, and sort each group
                    by value in column B
    ggABnCn-        Sort A-groups ordered by B numeric and C descending numeric

    ADVANCED
    g_100           Split input into 100-line chunks, sort each individually
    gM              Use 'sort -m' to merge sorted streams, each specified as a
                    filename
    gMB-            Merge sorted streams on column B descending (streams must be
                    sorted this way too)

    $ ni //ni/conf r/^row/ _
                    Show current sort parameters, including compression,
                    parallelism, and buffer size (used only with GNU coreutils
                    sort)

    $ export NI_ROW_SORT_BUFFER=64M
                    Set maximum allowed memory for a sort operation: anything
                    larger will be compressed and mergesorted on disk, which
                    creates IO overhead


DATA CLOSURES (ni //help/closure)
    ::foo[...]      Store the output of 'ni ...' into "foo" memory dataclosure
    :@foo[...]      Store the output of 'ni ...' into "foo" file dataclosure
    //:foo          Output contents of "foo" memory dataclosure
    //@foo          Output contents of "foo" file dataclosure

    Data closures move across SSH and Hadoop boundaries, although you may
    overflow memory if you store large values. Data closure contents are also
    accessible to stream transformation code, e.g. p'foo()'.

    Example:

    $ ni ::words[ /usr/share/dict/words ] //help FW Z1 riA[ //:words ] g c O
    $ ni ::words /usr/share/dict/words //help FWZ1riA//:words gcO


STREAM TRANSFORMATION (ni //help/stream)
    +[...]          Append lines from 'ni ...'
    ^[...]          Prepend lines from 'ni ...'
    %[...]          Interleave lines with 'ni ...', as data is available
    %4[...]         Interleave four lines for every one from 'ni ...'
    %-4[...]        Interleave one line for every four from 'ni ...'
    =[...]          Duplicate stream into 'ni ... > /dev/null'

    :               Copy data verbatim
    e'...'          Filter stream with '...' shell command
    e[grep -v foo]  Filter stream with 'grep -v foo' shell command

    S4[...]         Run four copies of pipeline section '...', shard data across
                    them, and merge outputs -- note that this reorders your data

    ADVANCED
    p'...'          Run Perl code '...' on each input line (see //help/perl)
    pR[...]         Preload Perl code generated by 'ni ...' (see //help/perl)

    y'...'          Run Python code '...' on each input line (see //help/python)
    yR[...]         Preload Python code generated by 'ni ...'
                    (see //help/python)
    yI'...'         Identical to i'...', but indents Python code correctly when
                    used for multiline strings

    m'...'          Run Ruby code '...' on each input line (see //help/ruby)
    l'...'          Run Lisp code '...' on each input line (see //help/lisp)
    js'...'         Run NodeJS code '...' on each input line (documentation TBD)
    c99'...'        Compile '...' as C99 and run the result on entire stream
                    (see //help/c)
    c++'...'        Compile '...' as C++ and run the result on entire stream
                    (requires 'c++' compiler; see //help/c)

    Bd64M           Copy stream through a 64M disk-backed FIFO

    shost[...]      Run pipeline section '...' on 'host' via SSH (ni will
                    self-install in memory, and data closures are forwarded)

    See also //help/binary to parse non-text streams.


FUNCTIONS AND LET-BINDINGS (ni //help/fn)
    f[%x %y : ...]          For each line of input, bind %x and %y as TSV and
                            run 'ni ...' with values substituted for %x and %y
    l[%x=5 %y=10 : ...]     Replace %x with 5 and %y with 10 in 'ni ...'
    fx8[%x : ...]           Use 'xargs -P8' to run 8 parallel 'ni ...'
                            processes, each a single f[] from the input

    EXAMPLES
    f[%f : i%f \<wcl]       Filenames -> line counts
    fx8[%f : i%f \<wcl]     Same, but read 8x at a time

    Note that you can call your arguments pretty much whatever you want to. The
    % prefix is optional; it just prevents your args from colliding with ni
    operators.

    Also note that ni parses the function body only once. This means your
    function arguments need to occur in positions where they don't change how
    your function is parsed; for example '%f' as a filename needs to be written
    as 'i%f \<' rather than directly.

    ni --explain       ->  explain without let-expansion
    ni --explain-meta  ->  explain after let-expansion (and other expansions)

    Also also note that because of xargs, fxN[] is subject to write corruption
    for large outputs. It's safer to have fxN[] output filenames and read them
    with \< or \<# -- for example, 'fx4[%f : ... \>] \<#'.


PERL STREAM CODE (ni //help/perl)
    Used both by p'...' and rp'...'.

    Perl stream processors run in a loop that invokes your code once per input
    line. You can use BEGIN and END blocks for cross-row state, or use multiline
    functions to read blocks of lines.

    FUNCTIONS
        a() .. l()          Values in columns A-L on current row
        F_(@indexes)        Values in indexed columns, or all if @indexes == ()
        $_                  Current line, with trailing newline
        FR($i)              All fields inclusive-rightwards from column $i
        FT($i)              All fields exclusive-until column $i
        FM()                Index of rightmost column on this line

        r(@values)          Write an output row of TSV @values, return ()

    MULTILINE FUNCTIONS
        Note that these move the current-line context forward, so a() .. l()
        will reflect the last-read line -- not the one you started with. It's
        common to say 'my $x = a; ...' when reading ahead.

        reA() .. reL()      Read while Equivalent along A .. (A-L) -- returns a
                            list of lines
        a_(@ls) .. l_(@ls)  Extract one column of data from a list of lines
        ab_(@ls) .. kl_()   Extract two columns of data with values interleaved

        rw{/foo/}           Read and return list of lines that satisfy /foo/
        ru{/foo/}           Read and return list of lines until the next one
                            satisfies /foo/

        rl($n = 1)          Advance and return $n lines ahead of the current one
        pl($n)              Peek and return $n lines ahead of the current one
                            (does not update a() .. l())

    UTILITY FUNCTIONS
        Below is an incomplete list; use 'ni --inspect' and explore the
        'core/pl' library for source definitions.

        rf($filename)       Read file into string, return it
        rfl($filename)      Read file into list of lines, return them
        ri(my $var, "< $f") Read file $f into $var
        ri(my $var, "ls |") Read output of "ls" into $var
        wf($f, $contents)   Write string $contents into file $f
        af($f, $contents)   Append string $contents to file $f

        je($thing)          JSON-encode a value ($thing can be a ref)
        jd($str)            JSON-decode a value into a Perl scalar

        tpe($ts =~ /\d+/g)  Time pieces to epoch (YmdHMS ordering)
        tep($e)             Time epoch to pieces (YmdHMS)
        tef($e)             Time epoch to formatted

        max(@values)        Returns maximum value under numeric comparison
        min(@values)
        maxstr(@values)     Returns maximum value under string comparison
        minstr(@values)

        any($f, @xs)        True iff $f->($x) for any $x in @xs
        all($f, @xs)        True iff $f->($x) for all $x in @xs

        argmax($f, @xs)     Returns $x from @xs maximizing $f->($x)
        argmin($f, @xs)
        indmax($f, @xs)     Returns $i from 0..$#xs maximizing $f->($xs[$i])
        indmin($f, @xs)

        sum(@values)        Math utils; see core/pl/math.pm in 'ni --inspect'
        prod(@values)       for more definitions
        mean(@values)
        median(@values)
        uniq(@values)
        var(@values)        Variance
        std(@values)        sqrt(var(@values))
        clip($l, $u, @xs)   Returns @xs, but clips all values to range [$l, $u]
        linspace(a, b, n)   Returns N evenly spaced values spanning [a, b]


    EXAMPLES
        Many more examples in //help/ex2 .. //help/ex6.

        p'a + b'            Add the first two columns of data
        p'r "foo", a, 5'    For each input row, write (foo, a, 5) as output
        p'length $_'        Return the length of each input line
        p'r a + 1, FR 1'    Add 1 to column A, return all other columns
                            unmodified

        p'my @ls = rea;     Read all lines whose A-column value is the same...
          sum(b_(@ls))'     ...and print the sum of that group's B column

        p'r rw{/^a/}'       Read all lines While /^a/ matches, then output them
                            on a single row
        p'r ru{/^a/}'       Read all lines Until /^a/ matches
        p'r rw{1}'          Read all lines in the entire stream
        p'a > 5 ? r a : ()' Write cell A for rows for which it's larger than 5

        p'r F_(4, 5)'       Write fields 4 and 5 on a row -- same as p'r e, f'
        p'F_(4, 5)'         Write fields 4 and 5 on separate lines
        pF_                 Idiom to flatten each row vertically

        ::dict[...] \       Store a stream into the ::dict data closure...
        p'^{%d = ab_ dict}  ...within a BEGIN block (^{}), parse cols A and B
                               from ::dict into a hash
          r a, $d{+a}'      ...for each row, write cell A and its hash
                               association


MATRIX TRANSFORMATION (ni //help/matrix)
    Y               Dense to sparse (each cell becomes row, col, val)
    X               Sparse to dense
    Z4              Reflow cells to be 4-wide on each row
    ZB              Flatten (a, b, c, d, e) into (a,b,c), (a,b,d), (a,b,e)
    Z^B             Invert ZB: collect (a,b,c), (a,b,d), (a,b,e) -> (a,b,c,d,e)

    N'x = x + 1'    Read whole stream into numpy matrix, use 'x = x + 1' as
                    Python code to transform matrix, write resulting matrix to
                    stream

    NA'x = abs(fft.fft(x))'
                    Read groups of rows having the same column-A value; for each
                    group, read into a numpy matrix, transform with specified
                    code, and write to stream -- keeping the column-A prefix

    Note that you can write multiline Python code; ni will infer the correct
    indentation and adjust accordingly.

    If you're working with large binary matrices, by'' is likely to be more
    efficient than N''.


BINARY PACKING (ni //help/binary)
    bf<template>        Read fixed-length rows with pack() <template>
    bf^<template>       Read TSV and emit fixed-length rows with pack()

    bp'...'             Run '...' Perl code over binary data
    by'...'             Run '...' Python code over binary data

    Use 'perldoc -f pack' for a full list of template elements. Note that bf
    handles only fixed-length templates: 'n/a' won't work, for example. If you
    need to unpack variable-length records, use the 'rp' (read-packed) function
    in bp'...', which uses buffered readahead:

    $ ni n10 bf^n/a bp'r rp"n/a"'     # NB: bf^ allows n/a; bf does not

    Note that by'' doesn't preload NumPy the way N'' does; its only imports are
    "os" and "sys".

    Also note that you _must_ use sys.stdin.buffer when reading binary data; if
    you use sys.stdin.read() directly, its own buffering will cause premature
    EOF, potentially causing your code not to see the last N bytes of data.

    by'' is a work in progress.

    BINARY PERL FUNCTIONS
        bi()              Return current stream offset in bytes
        available()       True if stream is not at EOF
        rp($packstring)   Read packed values, returning a list
        rb($nbytes)       Read exactly $nbytes bytes into a string
        pb($nbytes)       Peek (but don't consume) exactly $nbytes bytes
        wp($pack, @xs)    Pack @xs using $pack template, then write binary
        ws($data)         Write $data as binary, return ()

    FORMAT-SPECIFIC FUNCTIONS
        rppm()            Read PPM binary header: ($bytes, $magic, $w, $h, $max)


GNUPLOT
    G<col><term><cmd>  Use gnuplot to visualize data
    G:<term><cmd>      Plot data in a single group
    G:W%l              Plot one or two-column data with lines, interactively
    G*W                Plot all columns of data, keyed by col A on the X axis

    term can be:
      X  (x11)
      Q  (Qt)
      W  (Wx)
      J  (jpeg)
      PC (pngcairo)
      P  (png)

    cmd is a verbatim gnuplot command, with these shorthands:
      %l        plot "-" with lines
      %d        plot "-" with dots
      %i        plot "-" with impulses
      %v        plot "-" with vectors
      %t'...'   title "..."
      %u'...'   using ...

    G<col>, e.g. GA, causes gnuplot to be run multiple times -- one per group of
    rows for which column A is the same. This is useful when animating data.

    jpeg and png terminals will create image outputs on stdout, concatenated if
    gnuplot is run multiple times. ffmpeg can accept these concatenated image
    streams as input for video assembly. For example, to create an animated
    plot:

    $ ni n100,L p'r a, sin(a*$_/100) for 0..1000' GAP%l IVavi \>animated.avi

    If you're looping gnuplot with a column spec, ni sets a gnuplot variable
    called KEY that contains the current group value. You can use this by
    writing gnuplot code longhand:

    $ ni n100,L p'r a, sin(a*$_/100) for 0..1000' \
         GAP'set title "coefficient = " . KEY;
             plot "-" with lines' IVavi \>animated-title.avi

    NOTE: older versions of ffmpeg had a bug in the PNG image2pipe reader;
    version 4.2.4 (and possibly earlier) works correctly.


MEDIA
    yt://oHg5SJYRHA0      Stream video from youtube using youtube-dl
    v4l2:///dev/video0    Stream video from /dev/video0 v4l2 device
    x11grab://:0@640x480  Stream video from X11 display :0, clipped at 640x480
    m3u://https://...     Stream video from M3U playlist using ffmpeg

    VP                    Play video stream using ffplay
    VIppm                 Convert video to concatenated stream of PPM images
    VImjpeg               Convert video to concatenated stream of JPGs
    VIpng@1920x1080       Convert video and downsample to 1920x1080 resolution

    AE<mediaspec>         Use ffmpeg to discard video, emit audio as <mediaspec>
    IV<mediaspec>         Convert concatenated images to video (some older
                          ffmpegs fail if you use PNGs as input)

    I[...]                Split a stream of concatenated PNG, BMP, or PPM
                          images, transform each with 'ni ...'

    IC[init][fold][emit]  Left-fold a stream of concatenated PNG, BMP, or PPM
                          images using ImageMagick 'convert' (see below)

    <mediaspec> describes the container format, codec, and bitrate. The
    following examples are valid:

      IVavi                   AVI container format, default codec + bitrate
      IVgif                   GIF animated image
      IVmatroska/libvpx       Matroska with VPX codec, default bitrate
      AEogg/libvorbis/224k    Ogg container, vorbis audio codec, 224k bitrate

    m3u:// defaults to FLV, but you can specify the target media container, e.g.
    m3u+gif://URL. This may be required if the codec doesn't work with FLV.

    IC[][][] is a disk-intensive way to mix data between images within a
    sequence. It works like this:

      image 0 | convert $init > reduced.png
      while (more images)
        next image | convert reduced.png $fold > reduced.png

    Each time reduced.png is written, 'convert reduced.png $emit png:-' is run
    to emit a transformed version of it to stdout. This becomes the output image
    stream.

    IC's [] blocks are all 'convert' command-line argument lists. [init] can be
    written as : to specify no transformation. For example, to blur/fade:

      IC: [-blur 1x1 - -compose blend -define compose:args=100,98 -composite] \
          [-resize 1920x1080]


C99 JIT (ni //help/c)
    c99'C source'   Compile C source to executable, then pipe stream through it

    The c99'' operator will compile a C99 program immediately before using it as
    a stream filter. Because the C99 program remains on disk, your program
    should unlink itself by deleting argv[0].

    Your C program will have normal stdin/stdout/stderr IO available; there is
    no input preprocessing or line-splitting. Indentation is inferred as for
    Python.

    EXAMPLES
    ni c99'#include <stdio.h>
           #include <stdlib.h>
           int main(int argc, char **argv)
           {
             unlink(argv[0]);
             printf("hi!\n");
             return 0;
           }'


HASKELL JIT
    hs'HS source'   Use /usr/bin/env stack to run Haskell source, then pipe
                    stream through it

    This requires Haskell Stack to be runnable with "/usr/bin/env stack". Like
    C99 JIT, the Haskell program has stdin/stdout/stderr IO. Indentation is
    inferred as for Python.

    EXAMPLES
    ni hs'#!/usr/bin/env stack
          -- stack --resolver lts-18.3 script
          main :: IO ()
          main = putStrLn "hi!"'