theorem Th49:
  card {x1,x2} <= 2
proof
A1: {x1,x2} = {x1} \/ {x2} & 1+1 = 2 by ENUMSET1:1;
  card {x1} = 1 & card {x2} = 1 by CARD_1:30;
  hence thesis by A1,Th42;
end;
