code: protected void add(GroupMember member) {
  
          Assert.notNull(member, "member");
  
          if (members.add(member)) {
              member.onAdded();
          }
  
      }
docs: Attempts to add the given member to this expansion point. The member will not be added if
  an equivalent entry (according to its equals() method) already exists. If added, the member's
  {@link GroupMember#onAdded()} method will be called.
  
  @param member The member to be added. Must not be null.
  
  @throws IllegalArgumentException if {@code member} is null.