reserve x,x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,y for object, X,Z for set;

theorem Th6:
  { 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 Th3;
end;
