theorem Th61:
  { x1,x2,x3,x4 } = { x1,x2,x4,x3 }
proof
  thus { x1,x2,x3,x4 } = { x1 } \/ { x2,x3,x4 } by Th4
    .= { x1 } \/ { x2,x4,x3 } by Th57
    .= { x1,x2,x4,x3 } by Th4;
end;
