Ambiguous arguments

In these lines of the grammar, it is unclear whether, while in arguments, `a (b)` are two separate arguments, or a function call:
```
COMMA? ::= ,
COMMA? ::= ''

ARGS? ::= EXPR ARG_NEXT*
ARGS? ::= ''
ARG_NEXT* ::= COMMA? EXPR ARG_NEXT*
ARG_NEXT* :: ''
```