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

theorem
  { x1,x2,x3,x4,x5,x6 } = { x1,x2,x3,x4,x5 } \/ { x6 }
proof
  thus { x1,x2,x3,x4,x5,x6 } = { x1,x2,x3 } \/ { x4,x5,x6 } by Lm4
    .= { x1,x2,x3 } \/ ({ x4,x5 } \/ { x6 }) by Th3
    .= { x1,x2,x3 } \/ { x4,x5 } \/ { x6 } by XBOOLE_1:4
    .= { x1,x2,x3,x4,x5 } \/ { x6 } by Lm3;
end;
