theorem :: ZF_LANG1:15

for p1, p2 being ZF-formula
for x1, x2, y1, y2, z1, z2 being Variable st All (x1,y1,z1,p1) = All (x2,y2,z2,p2) holds
( x1 = x2 & y1 = y2 & z1 = z2 & p1 = p2 )