protect imported predicates from being overwritten (re: issue #28)

src/prolog/io.rs

@@ -267,7 +267,7 @@ impl fmt::Display for EvalError {
             &EvalError::ModuleDoesNotContainExport => write!(f, "module does not contain claimed export."),
             &EvalError::QueryFailure => write!(f, "false."),
             &EvalError::QueryFailureWithException(ref e) => write!(f, "{}", error_string(e)),
-            &EvalError::ImpermissibleEntry(ref msg) => write!(f, "cannot overwrite builtin {}.", msg),
+            &EvalError::ImpermissibleEntry(ref msg) => write!(f, "cannot overwrite {}.", msg),
             &EvalError::OpIsInfixAndPostFix =>
                 write!(f, "cannot define an op to be both postfix and infix."),
             &EvalError::NamelessEntry => write!(f, "the predicate head is not an atom or clause."),

src/prolog/machine/mod.rs

@@ -190,12 +190,10 @@ impl Machine {
                          -> EvalSession
     {
         match self.code_dir.get(&(name.clone(), arity)) {
-            Some(&CodeIndex (ref idx)) =>
-                if idx.borrow().1 == clause_name!("builtin") {
+            Some(&CodeIndex (ref idx)) if idx.borrow().1 != clause_name!("user") =>
                     return EvalSession::from(EvalError::ImpermissibleEntry(format!("{}/{}",
                                                                                    name,
-                                                                                   arity)))
-                },
+                                                                                   arity))),                
             _ => {}
         };