let x1, x2, x3, x4 be object ; :: thesis: card {x1,x2,x3,x4} <= 4
card {x2,x3,x4} <= 3 by Th50;
then A1: 1 + (card {x2,x3,x4}) <= 1 + 3 by XREAL_1:7;
( card {x1} = 1 & {x1,x2,x3,x4} = {x1} \/ {x2,x3,x4} ) by CARD_1:30, ENUMSET1:4;
then card {x1,x2,x3,x4} <= 1 + (card {x2,x3,x4}) by Th42;
hence card {x1,x2,x3,x4} <= 4 by A1, XXREAL_0:2; :: thesis: verum