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
  a 'nor' (b 'nand' c) = 'not' (a 'or' 'not' b 'or' 'not' c)
proof
  thus a 'nor' (b 'nand' c) =a 'nor' 'not' (b '&' c) by th1
    .='not' (a 'or' 'not' (b '&' c)) by Th2
    .='not' (a 'or' ('not' b 'or' 'not' c)) by BVFUNC_1:14
    .='not' (a 'or' 'not' b 'or' 'not' c) by BVFUNC_1:8;
end;
