code: function lengthOf(int $value = null): self {
      if ($value === null) return $this->setFlag('length');
  
      if (is_string($this->target)) {
        $constraint = equalTo($value);
        $target = mb_strlen($this->target);
      }
      else {
        $constraint = countOf($value);
        $target = $this->target;
      }
  
      return $this->expect($target, $constraint);
    }
docs: Reports an error if the length of the target is not the expected one.
  @param int $value The expected length.
  @return self This instance.