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