[jolmg]

Looking back at it, I forgot to allow spacing between parentheses and inner expression

  between(lit("("), lit(")"), spaced(expr)),

and using space as an operator prevents `(foo)(bar)` from being parsed as a call.

This is better, just adjacent expressions with optional in-between whitespace:

  define_combinator(:call_op) {|left, right| group(left < ws, right) }

Too late to edit, though. Oh well.