let p1, p2 be ZF-formula; :: thesis: for x1, y1, z1, x2, y2, z2 being Variable st Ex x1,y1,z1,p1 = Ex x2,y2,z2,p2 holds
( x1 = x2 & y1 = y2 & z1 = z2 & p1 = p2 )
let x1, y1, z1, x2, y2, z2 be Variable; :: thesis: ( Ex x1,y1,z1,p1 = Ex x2,y2,z2,p2 implies ( x1 = x2 & y1 = y2 & z1 = z2 & p1 = p2 ) )
assume Ex x1,y1,z1,p1 = Ex x2,y2,z2,p2 ; :: thesis: ( x1 = x2 & y1 = y2 & z1 = z2 & p1 = p2 )
then ( x1 = x2 & Ex y1,z1,p1 = Ex y2,z2,p2 ) by ZF_LANG:51;
hence ( x1 = x2 & y1 = y2 & z1 = z2 & p1 = p2 ) by Th20; :: thesis: verum