let x1, x2, y2, x3 be set ; :: thesis: [:{x1},{x2,y2},{x3}:] = {[x1,x2,x3],[x1,y2,x3]}
thus [:{x1},{x2,y2},{x3}:] = [:[:{x1},{x2,y2}:],{x3}:] by ZFMISC_1:def 3
.= [:{[x1,x2],[x1,y2]},{x3}:] by ZFMISC_1:36
.= {[x1,x2,x3],[x1,y2,x3]} by ZFMISC_1:36 ; :: thesis: verum