theorem
  ex p,q,r,s st p on B & q on B & r on B & s on B & p,q,r,s
  are_mutually_distinct
proof
  ex a,b,c,d,A st a on A & b on A & c on A & d on A & a,b,c,d
  are_mutually_distinct by Th16;
  hence thesis by Th13;
end;
