public void visit(OpConditional opCond) {
   Table left = eval(opCond.getLeft());
   // Ref engine - don;'t care about efficiency
   Table right = eval(opCond.getRight());
   Table table = evaluator.condition(left, right);
   push(table);
 }
 // Modularize.
 private static Op transformFilterConditional(
     ExprList exprs, Set<Var> varScope, OpConditional opConditional) {
   // Any filters that depend on no variables.
   Op op = insertAnyFilter(exprs, varScope, null);
   Op left = opConditional.getLeft();
   left = transform(exprs, varScope, left);
   Op right = opConditional.getRight();
   op = new OpConditional(left, right);
   op = insertAnyFilter(exprs, varScope, op);
   return op;
 }