How do I properly nest then/or/atLeast parsers? about parsimmon HOT 3 CLOSED

x
  .then(
    notFollowedBy(string("*"))
      .then(r.Inline)
      .or(r.Strong)
  )
  .atLeast(1);

function lookbehind(p, ofs) {
  if (ofs === undefined) {
    ofs = -1;
  }
  return Parser((input, i) => {
    const charI = i - ofs;
    if (charI < 0) {
      return makeSuccess(i, '');
    }
    const res = p._(input, charI);
    res.index = i;
    return res;
  });
}

function EmphasisUnderline(r) {
  const lfdr = string('_')
    .notFollowedBy(whitespace)
    .then(alt(
      notFollowedBy(r.PunctuationChar),
      lookahead(r.PunctuationChar).then(lookbehind(alt(whitespace, r.PunctuationChar), 2))
    ));
  
  const lfdrNotRfdr = lfdr
    .then(
      lookbehind(whitespace, 2)
      .or(
        lookbehind(r.PunctuationChar, 2)
        .lookahead(alt(whitespace, eof, r.PunctuationChar))
      )
    );

  const rfdr = string('_')
    .notFollowedBy(lookbehind(whitespace, 2))
    .then(alt(
      notFollowedBy(lookbehind(r.PunctuationChar, 2)),
      lookbehind(r.PunctuationChar, 2).lookahead(alt(whitespace, eof, r.PunctuationChar))
    ));
  
  const rfdrNotLfdr = rfdr
    .then(
      lookahead(alt(whitespace, eof))
      .or(
        lookahead(r.PunctuationChar)
        .then(lookbehind(alt(whitespace, r.PunctuationChar), 2)))
    );
  
  return (
    lfdrNotRfdr
      .then(
        notFollowedBy(rfdrNotLfdr).then(r.Inline).atLeast(1)
      )
      .map(children => ({
        type: 'emphasis',
        children,
      }))
      .skip(rfdrNotLfdr)
  );
}