reserve u,v,x,x1,x2,y,y1,y2,z,p,a for object,
        A,B,X,X1,X2,X3,X4,Y,Y1,Y2,Z,N,M for set;

theorem
  union {{x},{y}} = {x,y}
proof
  thus union {{x},{y}} = {x} \/ {y} by Lm15
    .= {x,y} by ENUMSET1:1;
end;
