reserve p,p1,p2,q,r,F,G,G1,G2,H,H1,H2 for ZF-formula,
  x,x1,x2,y,y1,y2,z,z1,z2,s,t for Variable,
  a,X for set;
reserve M for non empty set,
  m,m9 for Element of M,
  v,v9 for Function of VAR,M;
reserve i,j for Element of NAT;

theorem Th161:
  G1 'or' G2 = (H1 'or' H2)/(x,y) iff G1 = H1/(x,y) & G2 = H2/(x, y)
proof
  'not' G1 = ('not' H1)/(x,y) & 'not' G2 = ('not' H2)/(x,y) iff 'not' G1
  '&' 'not' G2 = ('not' H1 '&' 'not' H2)/(x,y) by Th158;
  hence thesis by Th156;
end;
