title: I want to check if std_logic_vector contains negative integer
body: <p>How can I check with if (...) then ... end if; construction if std_logic_vector variable holds the bits of a negative number? If it is negative, I have to assign it a zero value.
  I have :</p>
  
  <pre><code>signal sum : std_logic_vector (15 downto 0);
  sum&lt;= (...);
  if (...) then
     sum&lt;=x"00";
  end if;
  </code></pre>
  
  <p>Thank you!</p>