How can I allow keywords as identifiers?

This grammar allows "if if call call;" and "call if;".

grammar Pred;

prog: stat+ ;

stat: keyIF expr stat
    | keyCALL ID ';'
    | ';'
    ;

expr: ID
    ;

keyIF : {input.LT(1).getText().equals("if")}? ID ;

keyCALL : {input.LT(1).getText().equals("call")}? ID ;

ID : 'a'..'z'+ ;
WS : (' '|'\n')+ {$channel=HIDDEN;} ;