theorem Th4:
  { x1,x2,x3,x4 } = { x1 } \/ { x2,x3,x4 }
proof
  thus { x1,x2,x3,x4 } = { x1,x2 } \/ { x3,x4 } by Lm2
    .= { x1 } \/ { x2 } \/ { x3,x4 } by Th1
    .= { x1 } \/ ({ x2 } \/ { x3,x4 }) by XBOOLE_1:4
    .= { x1 } \/ { x2,x3,x4 } by Th2;
end;
