public Object visit(ConcatenateExpression node) {
            Expression left = node.getLeftExpression();
            Expression right = node.getRightExpression();

            check(left);
            check(right);

            left.convertTo(Type.NON_NULL_STRING_TYPE, false);
            right.convertTo(Type.NON_NULL_STRING_TYPE, false);
            node.setType(Type.NON_NULL_STRING_TYPE);

            return null;
        }