theorem Th50:
  card {x1,x2,x3} <= 3
proof
  card {x2,x3} <= 2 by Th49;
  then
A1: 1 + card {x2,x3} <= 1+2 by XREAL_1:7;
  card {x1} = 1 & {x1,x2,x3} = {x1} \/ {x2,x3} by CARD_1:30,ENUMSET1:2;
  then card {x1,x2,x3} <= 1 + card {x2,x3} by Th42;
  hence thesis by A1,XXREAL_0:2;
end;
