let UN be Universe; :: thesis: for u, v, w being Element of UN holds {u,v,w} is Element of UN
let u, v, w be Element of UN; :: thesis: {u,v,w} is Element of UN
{u,v,w} = {u,v} \/ {w} by ENUMSET1:3;
hence {u,v,w} is Element of UN ; :: thesis: verum