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;
