let X, Y be set ; :: thesis: union {X,Y,{}} = union {X,Y}
thus union {X,Y,{}} = union ({X,Y} \/ {{}}) by ENUMSET1:3
.= (union {X,Y}) \/ (union {{}}) by ZFMISC_1:78
.= (union {X,Y}) \/ {} by ZFMISC_1:25
.= union {X,Y} ; :: thesis: verum