diff --git a/src/language/types.rs b/src/language/types.rs
index a1f68dd1..da453bbb 100644
--- a/src/language/types.rs
+++ b/src/language/types.rs
@@ -131,7 +131,7 @@ impl<'i> Identifier<'i> {
     }
 }
 
-#[derive(Eq, Debug)]
+#[derive(Clone, Copy, Eq, Debug)]
 pub struct External<'i> {
     pub value: &'i str,
     pub span: Span,
diff --git a/src/problem/messages.rs b/src/problem/messages.rs
index e1397bf6..95fa7add 100644
--- a/src/problem/messages.rs
+++ b/src/problem/messages.rs
@@ -1137,10 +1137,7 @@ functions calls:
             function: Identifier { value: name, .. },
         } => (
             format!("Unknown function {}()", name),
-            format!(
-                "The function {}() is neither builtin nor provided by the selected domain.",
-                name
-            ),
+            "The function is neither builtin nor provided by the selected domain.".to_string(),
         ),
     }
 }
diff --git a/src/program/types.rs b/src/program/types.rs
index 619d1f26..a0515366 100644
--- a/src/program/types.rs
+++ b/src/program/types.rs
@@ -145,11 +145,13 @@ pub struct Invocable<'i> {
 /// subroutine into `Program.subroutines`; the resolve pass walks the
 /// translated tree replacing matching `Unresolved` references with
 /// `Resolved`. Names that don't match any declared subroutine become a
-/// translation error.
+/// translation error. `Deferred` references (ie URLs) are left as-is; they
+/// are resolved at a later phase or at runtime.
 #[derive(Debug, Eq, PartialEq)]
 pub enum SubroutineRef<'i> {
     Unresolved(language::Identifier<'i>),
     Resolved(SubroutineId),
+    Deferred(language::External<'i>),
 }
 
 /// Lowered form of `language::Function`. Functions live in a separate
diff --git a/src/runner/runner.rs b/src/runner/runner.rs
index 046262c4..037a6b63 100644
--- a/src/runner/runner.rs
+++ b/src/runner/runner.rs
@@ -472,6 +472,11 @@ impl<'i, D: Driver> Runner<'i, D> {
                     .announce(&format!("<{}>", id.value));
                 Ok(Outcome::Done(Value::Unitus))
             }
+            SubroutineRef::Deferred(ext) => {
+                self.driver
+                    .announce(&format!("<{}>", ext.value));
+                Ok(Outcome::Done(Value::Unitus))
+            }
         }
     }
 
diff --git a/src/translation/translator.rs b/src/translation/translator.rs
index b61205bd..32a8e76d 100644
--- a/src/translation/translator.rs
+++ b/src/translation/translator.rs
@@ -745,7 +745,7 @@ impl<'i> Translator<'i> {
     fn translate_invocation(&mut self, invocation: &'i language::Invocation<'i>) -> Invocable<'i> {
         let target = match &invocation.target {
             language::Target::Local(id) => SubroutineRef::Unresolved(*id),
-            language::Target::Remote(_) => todo!("remote invocation target"),
+            language::Target::Remote(external) => SubroutineRef::Deferred(*external),
         };
         let arguments = match &invocation.parameters {
             Some(params) => params