reserve Y for non empty set,
  a,b,c,d for Function of Y,BOOLEAN;
reserve Y for non empty set,
  a,b,c for Function of Y,BOOLEAN;

theorem Th63:
  a 'xor' (b 'nor' c) = (a 'or' 'not' (b 'or' c)) '&' ('not' a 'or' b 'or' c)
proof
  thus a 'xor' (b 'nor' c) =a 'xor' 'not' (b 'or' c) by Th2
    .=(a 'or' 'not' (b 'or' c)) '&' ('not' a 'or' 'not' 'not' (b 'or' c)) by
BVFUNC_6:86
    .=(a 'or' 'not' (b 'or' c)) '&' ('not' a 'or' b 'or' c) by BVFUNC_1:8;
end;
