let x1, x2, x3, y2, y3 be object ; :: thesis: [:{x1},{x2,y2},{x3,y3}:] = {[x1,x2,x3],[x1,y2,x3],[x1,x2,y3],[x1,y2,y3]}
thus [:{x1},{x2,y2},{x3,y3}:] = [:[:{x1},{x2,y2}:],{x3,y3}:] by ZFMISC_1:def 3
.= [:{[x1,x2],[x1,y2]},{x3,y3}:] by ZFMISC_1:30
.= {[[x1,x2],x3],[[x1,x2],y3],[[x1,y2],x3],[[x1,y2],y3]} by Th17
.= {[x1,x2,x3],[x1,y2,x3],[x1,x2,y3],[x1,y2,y3]} by ENUMSET1:62 ; :: thesis: verum