cache ball terms before unifying in handle_ball/3 (#1608)

src/machine/system_calls.rs

@@ -4804,11 +4804,6 @@ impl Machine {
         }
     }
 
-    #[inline(always)]
-    pub(crate) fn erase_ball(&mut self) {
-        self.machine_st.ball.reset();
-    }
-
     #[inline(always)]
     pub(crate) fn get_ball(&mut self) {
         let addr = self.machine_st.store(self.machine_st.deref(self.machine_st.registers[1]));
@@ -4828,6 +4823,29 @@ impl Machine {
         };
     }
 
+    #[inline(always)]
+    pub(crate) fn push_ball_stack(&mut self) {
+        if self.machine_st.ball.stub.len() > 0 {
+            self.machine_st.ball_stack.push(
+                mem::replace(&mut self.machine_st.ball, Ball::new())
+            );
+        } else {
+            self.machine_st.fail = true;
+        }
+    }
+
+    #[inline(always)]
+    pub(crate) fn pop_ball_stack(&mut self) {
+        self.machine_st.ball_stack.pop();
+    }
+
+    #[inline(always)]
+    pub(crate) fn pop_from_ball_stack(&mut self) {
+        if let Some(ball) = self.machine_st.ball_stack.pop() {
+            self.machine_st.ball = ball;
+        }
+    }
+
     #[inline(always)]
     pub(crate) fn get_current_block(&mut self) {
         let n = Fixnum::build_with(i64::try_from(self.machine_st.block).unwrap());