if(index < tokens.size() && tokens.get(index) instanceof Comma) {
match(Comma.class);
matchKeyword("if");
Expr condition = parseCondition();
matchEndLine();
return new RuleDecl(lets,result,condition,sourceAttr(start,index-1));
} else {
return new RuleDecl(lets,result,null,sourceAttr(start,index-1));
}
}