[antlr-interest] Problems with syntactic predicates(?)

[Christian Gudrian]

Hello!

I want to write a grammar that matches equations like:

a=1
2=b
f_a(3)=4
5=f_b(6,7)
f_c(8,9)

This is my approach so far:

// -------------------------- 8< --------------------------

grammar test;

INT	:	( '0'..'9' )+ ;	
IDENT	:	( 'a'..'z' )+ ;
	
equation	
	:	( expression '=' ) => expression '=' expression
	|	function_call
	;

expression	
	:	( IDENT '(' ) => function_call
	|	INT
	|	IDENT
	;
	
function_call	
	:	IDENT '(' args? ')' ;
	
args	:	expression ( ',' expression )* ;

// ------------------------- >8 --------------------------

The ANTLRWorks debugger shows me, that the equation rule matches input  
of the form "a=1".  Everything involving function calls doesn't match,  
however; I get a NoViableAltException(0!=[null]) error in these  
cases.  I suspect it's my use of syntactic predicates which causes  
this behaviour, isn't it?  What's wrong?

Christian