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 Lm2
    .= { x1,x2 } \/ ({ x3,x4 } \/ { x5,x6,x7,x8,x9 }) by XBOOLE_1:4
    .= { x1,x2 } \/ { x3,x4,x5,x6,x7,x8,x9 } by Th17;
end;
