title: found '0' definitions of operator "+" in VHDL
body: <p>At first I wanna point out that this is my first attempt with VHDL so be kind. I want to read the X1 ... X4 inputs and produce the sum of the ones at the output. This my code</p>
  
  <pre><code>library IEEE;
  use IEEE.STD_LOGIC_1164.ALL;
  
  entity counter_of_aces is 
    Generic(N: integer := 3);
       port( X1, X2, X3, X4 : IN BIT; 
          count: out std_logic_vector(N-1 downto 0)); 
  end counter_of_aces;
  
  architecture behavioral of counter_of_aces is 
  signal counter : std_logic_vector(Ν-1 downto 0);
  begin 
    process (X1, X2, X3, X4) 
    begin
      counter &lt;= "0";
      if(X1='1' OR X2='1' OR X3='1' OR X4='1')then 
          counter &lt;= counter + "1"; --O counter λειτουργεί ως στοιχείο μνήμης 
      else
          counter &lt;= counter;
      end if; 
    end process; 
  end behavioral;
  </code></pre>
  
  <p>and I get the following errors</p>
  
  <pre><code>ERROR:HDLCompiler:69 - Line 11: &lt;í&gt; is not declared.
  ERROR:HDLCompiler:1731 - Line 17: found '0' definitions of operator "+", cannot determine exact overloaded matching definition for "+"
  ERROR:HDLCompiler:854 - Line 10: Unit &lt;behavioral&gt; ignored due to previous errors.
  </code></pre>
  
  <p>Which "i" is it referring to and what about the others? Thanks in advance. </p>