Prepare before typing the untyped

bytecode_interpreter.cpp

@@ -143,7 +143,6 @@ union Register{
 };
 
 enum{
-  REG_NULL,
   REG_STACK_POINTER,
   REG_INSTRUCTION_POINTER,
   REG_SPECIAL_COUNT,
@@ -186,7 +185,7 @@ struct Bc{
 //
 function Register_Index
 allocate_register(Bc *b){
-  Register_Index result = 0;
+  Register_Index result = -1;
   if(b->free_registers.len == 0)
     result = b->registers.addi({});
   else
@@ -208,6 +207,7 @@ release_register(Bc *b, Register_Index reg){
   }
 
   assert_msg(found, "Trying to release register that is not used");
+  b->free_registers.add(reg);
 }
 
 function Bc