code: function touchStart(element, clientX, clientY) {
    element[symbols.dragging] = true;
    element[startXSymbol] = clientX;
    element[previousXSymbol] = clientX;
    element[previousYSymbol] = clientY;
    element[deltaXSymbol] = 0;
    element[deltaYSymbol] = 0;
  }
docs: /*
  Invoked when the user has begun a touch operation.