def special_number?(literal)
      literal.is_a?(Sass::Script::Value::String) && literal.value =~ /(calc|var)\(/
    end
def special_number?(literal)
      literal.is_a?(Sass::Script::Value::String) && literal.value =~ /(calc|var)\(/
    end
Returns whether the literal is a special CSS value that may evaluate to a
 number, such as `calc()` or `var()`.

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