theorem Th26:
  (x+y).|.(x+y) = x.|.x + x.|.y + y.|.x + y.|.y
proof
  (x+y).|.(x+y) = x.|.(x+y) + y.|.(x+y) by Def11
    .= (x.|.x + x.|.y) + y.|.(x+y) by Th12
    .= (x.|.x + x.|.y) + (y.|.x + y.|.y) by Th12;
  hence thesis;
end;
