title: How to add std_logic using numeric_std
body: <p>Using numeric_std and vhdl93, I cant seems to figure out how to add a std_logic signal to a std_logic_vector.</p>
  
  <pre><code>library ieee;
  use ieee.numeric_std.all;
  
  signal in_a, out1: std_logic_vector(3 downto 0);
  signal s1 : std_logic;
  
  out1 &lt;= std_logic_vector(signed(in_a) + s1);
  </code></pre>