/**
  * Use the id for equals.
  *
  * @see java.lang.Object#equals(java.lang.Object)
  */
 @Override
 public boolean equals(Object obj) {
   if (!(obj instanceof AbstractDrawing)) return false;
   return id.equals(obj);
 }
 /**
  * Use the id for hash code.
  *
  * @see java.lang.Object#hashCode()
  */
 @Override
 public int hashCode() {
   return id.hashCode();
 }