theorem Th11:
  { 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 Th2
    .= { x1 } \/ ({ x2,x3 } \/ { x4,x5,x6 }) by XBOOLE_1:4
    .= { x1 } \/ { x2,x3,x4,x5,x6 } by Th8;
end;
