def var?(literal)
      literal.is_a?(Sass::Script::Value::String) && literal.value =~ /var\(/
    end
def var?(literal)
      literal.is_a?(Sass::Script::Value::String) && literal.value =~ /var\(/
    end
Returns true when the literal is a string containing a var().

 @param literal [Sass::Script::Value::Base] The value to check
 @return Boolean