I want to use a syntactic predicate, but only with one alternative.

Sometimes you use rules with an empty alternative with syntactic predicates. In R/BASIC a COLON could be used as an operator in an expression, or it could follow an expression to end a PRINT statment, so I needed a syntactic predicate to distinguish between

PRINT A:B
PRINT A:B:

What I wanted to do was:

concatenationExpr
    :  additiveExpr
       (COLON ~(NEWLINE)) =>
           ((COLON | TRICOLON) concatenationExpr)
    ;

concatenationExpr
    :  additiveExpr
       (    (COLON ~(NEWLINE)) =>
                ((COLON | TRICOLON) concatenationExpr)
       |
       )
    ;

This works fine, but you do have to be very aware of putting anything with an empty alternative into a loop because you might end up with an infinite loop. As a matter of fact the next thing I thought was that most concatenationExprs are right after another, so I could do this:

concatenationExpr
    :  additiveExpr
       (    (COLON ~(NEWLINE)) =>
               ((COLON | TRICOLON) concatenationExpr)
       |
       )*
    ;

This gave me the infinite loop when the syntactic predicate failed, and matched the empty alt over and over.