read from streams.

src/prolog/machine/or_stack.rs

@@ -1,5 +1,6 @@
 use prolog::machine::machine_indices::*;
 
+use std::mem;
 use std::ops::{Index, IndexMut};
 use std::vec::Vec;
 
@@ -74,6 +75,11 @@ impl OrStack {
         self.0.push(Frame::new(global_index, e, cp, attr_var_init_b, b, bp, tr, pstr_tr, h, b0, n));
     }
 
+    #[inline]
+    pub(crate) fn take(&mut self) -> Self {
+        OrStack(mem::replace(&mut self.0, vec![]))
+    }
+
     pub fn len(&self) -> usize {
         self.0.len()
     }