theorem Th32:
  { x1,x1,x2,x3,x4 } = { x1,x2,x3,x4 }
proof
  thus { x1,x1,x2,x3,x4 } = { x1,x1 } \/ { x2,x3,x4 } by Th8
    .= { x1 } \/ { x2,x3,x4 } by Th29
    .= { x1,x2,x3,x4 } by Th4;
end;
