Id  ::= [a-zA-Z_][a-zA-Z0-9_$?]*
exp ::= Id                            -- Identifier
      | Number                        -- Float literal e.g. -1.3e2
      | String                        -- String literal e.g. "o_O"
      | "native" String               -- Native binding
      | '(' ')'                       -- Unit
      | '(' exp+ ')'                  -- Block of application
      | '$' exp+                      -- Infix application
      | '{' (Id+ "->")? exp+ '}'      -- Abstraction à la Kotlin
      | "let" Id '=' exp+ "in" exp+   -- Binding
def ::= "let" Id '=' exp+
s0  ::= def* 

let leq   { l r t f -> native "leq"   }
let mult  { l r     -> native "mult"  }
let minus { l r     -> native "minus" }

let cond { c t f -> c t f () }

let fact { a ->
    cond (leq a 1)
         { 1 }
         { mult a $ fact $ minus a 1 }
}         

let _ = fact 12