theorem Th30:
  { x1,x1,x2 } = { x1,x2 }
proof
  thus { x1,x1,x2 } = { x1,x1 } \/ { x2 } by Th3
    .= { x1 } \/ { x2 } by Th29
    .= { x1,x2 } by Th1;
end;
