--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_decr.c	Thu Oct 15 19:55:15 2020 +0800
@@ -0,0 +1,34 @@
+#include "tommath_private.h"
+#ifdef BN_MP_DECR_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* Decrement "a" by one like "a--". Changes input! */
+mp_err mp_decr(mp_int *a)
+{
+   if (MP_IS_ZERO(a)) {
+      mp_set(a,1uL);
+      a->sign = MP_NEG;
+      return MP_OKAY;
+   } else if (a->sign == MP_NEG) {
+      mp_err err;
+      a->sign = MP_ZPOS;
+      if ((err = mp_incr(a)) != MP_OKAY) {
+         return err;
+      }
+      /* There is no -0 in LTM */
+      if (!MP_IS_ZERO(a)) {
+         a->sign = MP_NEG;
+      }
+      return MP_OKAY;
+   } else if (a->dp[0] > 1uL) {
+      a->dp[0]--;
+      if (a->dp[0] == 0u) {
+         mp_zero(a);
+      }
+      return MP_OKAY;
+   } else {
+      return mp_sub_d(a, 1uL,a);
+   }
+}
+#endif