theorem Th31:
  { x1,x1,x2,x3 } = { x1,x2,x3 }
proof
  thus { x1,x1,x2,x3 } = { x1,x1 } \/ { x2,x3 } by Lm2
    .= { x1 } \/ { x2,x3 } by Th29
    .= { x1,x2,x3 } by Th2;
end;
