No-op argument in backcall

[vendethiel]

Maybe same as #194 but , arg <- b compiles to b(function (arg) {});
(, arg) <- b compiles correctly

(also, would it be interesting to give the backcall the ability to be alone, so that we can return a function or something ?)

[satyr]

You can't omit parens there. Just use the formal syntax ((, arg) <- b) when ambiguous.

$ coco -bcs
    a
    , b <- c

    a
    (, b) <- c

c(function(a, b){
  a;
  return c(function(arg$, b){});
});