code: def transform_value(value, transform)
        case transform
        when nil
          value
        when Proc
          @parent.instance_exec(value, &transform)
        when Symbol
          value.public_send(transform)
        end
      end
docs: Apply a transformation to a value.
   @param value [Object] a value
   @param transform [nil, Proc, Symbol] a transform type
   @return [Object] the transformed value