let x1, x2, x3, y3 be set ; :: thesis: [:{x1},{x2},{x3,y3}:] = {[x1,x2,x3],[x1,x2,y3]}
thus [:{x1},{x2},{x3,y3}:] = [:[:{x1},{x2}:],{x3,y3}:] by ZFMISC_1:def 3
.= [:{[x1,x2]},{x3,y3}:] by ZFMISC_1:35
.= {[x1,x2,x3],[x1,x2,y3]} by ZFMISC_1:36 ; :: thesis: verum