code: public function when(bool $test, $decodings): self
      {
          if (!$test) {
              return $this;
          }
  
          if ($decodings instanceof \Closure) {
              return $decodings($this);
          }
  
          $decodings = $decodings instanceof Decoding ? [$decodings] : $decodings;
  
          return $this->push(...$decodings);
      }
docs: Push decodings if the truth test evaluates to true.
  
  @param bool $test
  @param Decoding|iterable|\Closure $decodings
  @return DecodingList