theorem
  [:{x1,y1},{x2,y2},{x3}:] = { [x1,x2,x3],[y1,x2,x3],[x1,y2,x3],[y1,y2,
  x3] }
proof
  thus [:{x1,y1},{x2,y2},{x3}:] = [:[:{x1,y1},{x2,y2}:],{x3}:] by
ZFMISC_1:def 3
    .= [:{[x1,x2],[x1,y2],[y1,x2],[y1,y2]},{x3}:] by Th17
    .= [:{[x1,x2],[x1,y2]} \/ {[y1,x2],[y1,y2]},{x3}:] by ENUMSET1:5
    .= [:{[x1,x2],[x1,y2]},{x3}:] \/ [:{[y1,x2],[y1,y2]},{x3}:] by ZFMISC_1:97
    .= { [[x1,x2],x3],[[x1,y2],x3]} \/ [:{[y1,x2],[y1,y2]},{x3}:] by
ZFMISC_1:30
    .= { [[x1,x2],x3],[[x1,y2],x3]} \/ {[[y1,x2],x3],[[y1,y2],x3] } by
ZFMISC_1:30
    .= { [x1,x2,x3],[x1,y2,x3],[y1,x2,x3],[[y1,y2],x3] } by ENUMSET1:5
    .= { [x1,x2,x3],[y1,x2,x3],[x1,y2,x3],[y1,y2,x3] } by ENUMSET1:62;
end;
