fix atomic_concat/3 to be backtrackable.

H/YapText.h

@@ -172,6 +172,18 @@ Yap_AtomicToListOfCodes(Term t0 USES_REGS)
   return out.val.t;
 }
 
+static inline Atom
+Yap_AtomicToAtom(Term t0 USES_REGS)
+{
+  seq_tv_t inp, out;
+  inp.val.t = t0;
+  inp.type = YAP_STRING_STRING|YAP_STRING_ATOM|YAP_STRING_INT|YAP_STRING_FLOAT|YAP_STRING_BIG|YAP_STRING_TERM;
+  out.type = YAP_STRING_ATOM;
+  if (!Yap_CVT_Text(&inp, &out PASS_REGS))
+    return 0L;
+  return out.val.a;
+}
+
 static inline size_t
 Yap_AtomToLength(Term t0 USES_REGS)
 {