theorem
  [:{x1},{x2},{x3},{x4}:] = { [x1,x2,x3,x4] }
proof
  thus [:{x1},{x2},{x3},{x4}:] = [:[:{x1},{x2}:],{x3},{x4}:] by Th38
    .= [:{[x1,x2]},{x3},{x4}:] by ZFMISC_1:29
    .= {[[x1,x2],x3,x4]} by Th25
    .= { [x1,x2,x3,x4] };
end;
