let x1, x2, x3 be object ; :: thesis: card {x1,x2,x3} <= 3
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 card {x1,x2,x3} <= 3 by A1, XXREAL_0:2; :: thesis: verum