Provide ability to reference current value of process pipeline as a source property

My mind was just blown. I was thinking about this again, this time thinking that I'd have a SuperGet class that would override Get. Then I saw that Get can already be coaxed into referencing the current value of the pipeline. From its transform method:

foreach ($properties as $property) {
      if ($property || (string) $property === '0') {
        $return[] = $row->get($property);
      }
      else {
        $return[] = $value;
      }
    }

That "else" case puts the current pipeline value into $return. When does that case run? When $property is null an empty string or an empty array. Those are essentially keywords that do the thing I was introducing with %pipeline So we already can do this! The snippet from the IS would be:

process:
  processed_value:
    -
      plugin: callable
      source: field_product
      callable: ucfirst
    -    
       plugin: concat
       source:
         - 'constants/i'
         - null

It's weird and not intuitive and hard to read, but it totally works!!!