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

theorem Th10:
  { x1,x2,x3,x4,x5 } = { x1,x2,x3,x4 } \/ { x5 }
proof
  thus { x1,x2,x3,x4,x5 } = { x1,x2,x3 } \/ { x4,x5 } by Lm3
    .= { x1,x2,x3 } \/ ({ x4 } \/ { x5 }) by Th1
    .= { x1,x2,x3 } \/ { x4 } \/ { x5 } by XBOOLE_1:4
    .= { x1,x2,x3,x4 } \/ { x5 } by Th6;
end;
