docs: function updatePen( penIndex, penColor = "0" ) {
      if ( !this._content ) return;
  
      let elementsToFill = this._content.elementsToFillPerColor[ penIndex ];
      if ( !Array.isArray( elementsToFill ) ) elementsToFill = [];
      let elementsToStroke = this._content.elementsToStrokePerColor[ penIndex ];
      if ( !Array.isArray( elementsToStroke ) ) elementsToStroke = [];
  
      updateColor( elementsToFill, elementsToStroke, penColor );
  }