Parser’ s bug? Parentheses in an "s"-type argument

I’m not sure it is a bug (do not know whether the syntax is
documented anywhere), but this definitely makes it harder to write a
readable code.

(19:23) gp > my(a=0,b=1);print(a"\t"(a+=b))
  ***   at top-level: my(a=0,b=1);print(a"\t"(a+=b))
  ***                                    ^-----------
  ***   not a function in function call

Thanks,
Ilya