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' (a '&' b) = 'not' (a '&' (a 'or' b))
proof
  thus a 'nor' (a '&' b) = 'not' (a 'or' a) 'or' 'not' (a 'or' b) by Th33
    .= 'not' (a '&' (a 'or' b)) by BVFUNC_1:14;
end;
