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

 Use \{#special_number?} in preference to this.

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