reserve v,x,x1,x2,x3,x4,y,y1,y2,y3,y4,z,z1,z2 for object,
  X,X1,X2,X3,X4,Y,Y1,Y2,Y3,Y4,Y5,
  Z,Z1,Z2,Z3,Z4,Z5 for set;
reserve p for pair object;
reserve R for Relation;
reserve xx1 for Element of X1,
  xx2 for Element of X2,
  xx3 for Element of X3;
reserve xx4 for Element of X4;

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;
