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,x7,x8,x9 } = { x1 } \/ { x2,x3,x4,x5,x6,x7,x8,x9 }
proof
  thus { x1,x2,x3,x4,x5,x6,x7,x8,x9 } = { x1,x2,x3,x4 } \/ { x5,x6,x7,x8,x9 }
  by Lm9
    .= ({ x1 } \/ { x2,x3,x4 }) \/ { x5,x6,x7,x8,x9 } by Th4
    .= { x1 } \/ ({ x2,x3,x4 } \/ { x5,x6,x7,x8,x9 }) by XBOOLE_1:4
    .= { x1 } \/ { x2,x3,x4,x5,x6,x7,x8,x9 } by Th24;
end;
