They work alongside Lisp-style ;; and {--}, without any #pragma.
Conceptually, a new syntax should be disabled by default
and activated using a special compiler option.
But now, we don't have an easy way to provide compiler options
in func-js, blueprint, etc.
Note, that introducing per-file #pragma is a wrong approach here,
since if we want to fire human-readable error on using '//' without pragma,
lexer should nevertheless work differently.
(this could be controlled by a launch option, but see above)