theorem Th25:
  [:{x1},{x2},{x3}:] = { [x1,x2,x3] }
proof
  thus [:{x1},{x2},{x3}:] = [:[:{x1},{x2}:],{x3}:] by ZFMISC_1:def 3
    .= [:{[x1,x2]},{x3}:] by ZFMISC_1:29
    .= { [x1,x2,x3] } by ZFMISC_1:29;
end;
