code: public Membership remove(Address old_member) {
          if(old_member != null) {
              synchronized(members) {
                  members.remove(old_member);
              }
          }
          return this;
      }
docs: Removes an member from the membership. If this member doesn't exist, no action will be
  performed on the existing membership
  
  @param old_member
  - the member to be removed