MaxPlus 2: compiling errors

The error message isn't fully correct, because there is no else clause to the clock edge condition. Quartus gives a better understandable message.

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Error (10818): Can't infer register for "n[0]" at mod_tr.vhd(22) because it does not hold its value outside the clock edge

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The code can be compiled, when you change it like this:

process (start,clk,clr)--contador
  variable n: integer range 0 to 4;for the 
begin
if clk'event and clk='1'then
   n:=contador;
   if start='1' or clr='1' then
      n:=0;
   elsif contador/=4 then 
      n:=n+1;
   elsif contador=4 then
     clr<='1';
   end if;
   contador <= n;
end if;
end process;

You can write the same logic without the variable n. I don't think, that the code works as expected, because the signal clr is only set but never reset.