code: function contain($value = null): self {
      if ($this->hasFlag('file')) return $this->expect(@file_get_contents($this->target), stringContains($value));
      return $this->expect($this->target, is_string($this->target) ? stringContains($value) : contains($value));
    }
docs: Reports an error if the target does not contain an element or a substring.
  @param mixed $value The value to find.
  @return self This instance.