theorem Th12:
  { 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 Th3
    .= { x1,x2 } \/ ({ x3 } \/ { x4,x5,x6 }) by XBOOLE_1:4
    .= { x1,x2 } \/ { x3,x4,x5,x6 } by Th4;
end;
