add permission error for attempts at redefining builtins, re: #50

src/prolog/compile.rs

@@ -124,7 +124,14 @@ fn compile_decl(wam: &mut Machine, tl: TopLevel, queue: Vec<TopLevel>) -> EvalSe
             EvalSession::from(ParserError::InvalidModuleDecl),
         _ => {
             let name = try_eval_session!(if let Some(name) = tl.name() {
-                Ok(name)
+                match ClauseType::from(name.clone(), tl.arity(), None) {
+                    ClauseType::Named(..) | ClauseType::Op(..) =>
+                        Ok(name),
+                    _ => {
+                        let err_str = format!("{}/{}", name.as_str(), tl.arity());
+                        Err(SessionError::ImpermissibleEntry(err_str))
+                    }
+                }
             } else {
                 Err(SessionError::NamelessEntry)
             });