Add Machine::run_input_once() which reads one goal from user input an…

…d runs it

src/machine/mod.rs

@@ -307,6 +307,10 @@ impl Machine {
         self.run_module_predicate(atom!("$toplevel"), (atom!("$repl"), 1));
     }
 
+    pub fn run_input_once(&mut self) {
+        self.run_module_predicate(atom!("$toplevel"), (atom!("run_input_once"), 0));
+    }
+
     pub fn set_user_input(&mut self, input: String) {
         self.user_input = Stream::from_owned_string(input, &mut self.machine_st.arena);
     }

src/toplevel.pl

@@ -430,3 +430,6 @@
     % is expected to be printed instead.
     ;  print_exception(E)
     ).
+
+run_input_once :-
+    catch(read_and_match, E, print_exception(E)).