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 'imp' (a 'nor' b) = 'not' (a 'or' a '&' b)
proof
  thus a 'imp' (a 'nor' b) ='not' (a '&' (a 'or' b)) by Th65
    .='not' (a '&' a 'or' a '&' b) by BVFUNC_1:12
    .='not' (a 'or' a '&' b);
end;
