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 } = { x1,x2,x3,x4,x5 } \/ { x6,x7 }
proof
  thus { x1,x2,x3,x4,x5,x6,x7 } = { x1,x2,x3,x4 } \/ { x5,x6,x7 } by Lm5
    .= { x1,x2,x3,x4 } \/ ({ x5 } \/ { x6,x7 }) by Th2
    .= { x1,x2,x3,x4 } \/ { x5 } \/ { x6,x7 } by XBOOLE_1:4
    .= { x1,x2,x3,x4,x5 } \/ { x6,x7 } by Th10;
end;
