let U be Universe; :: thesis: for x being set holds {x} is U -petit
let x be set ; :: thesis: {x} is U -petit
reconsider u = {} as Element of U by CLASSES2:56;
( card {x} = 1 & card {u} = 1 ) by CARD_1:30;
hence {x} is U -petit by CARD_1:5; :: thesis: verum