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diff --git a/gmp/mpz/combit.c b/gmp/mpz/combit.c
new file mode 100644
index 00000000..02ceaf08
--- /dev/null
+++ b/gmp/mpz/combit.c
@@ -0,0 +1,80 @@
+/* mpz_combit -- complement a specified bit.
+
+Copyright 2002, 2003 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library.
+
+The GNU MP Library is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 3 of the License, or (at your
+option) any later version.
+
+The GNU MP Library is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+License for more details.
+
+You should have received a copy of the GNU Lesser General Public License
+along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+
+void
+mpz_combit (mpz_ptr d, unsigned long int bit_index)
+{
+  mp_size_t dsize = ABSIZ(d);
+  mp_ptr dp = LIMBS(d);
+
+  mp_size_t limb_index = bit_index / GMP_NUMB_BITS;
+  mp_limb_t bit = ((mp_limb_t) 1 << (bit_index % GMP_NUMB_BITS));
+
+  if (limb_index >= dsize)
+    {
+      MPZ_REALLOC(d, limb_index + 1);
+      dp = LIMBS(d);
+
+      MPN_ZERO(dp + dsize, limb_index + 1 - dsize);
+      dsize = limb_index + 1;
+    }
+
+  if (SIZ(d) >= 0)
+    {
+      dp[limb_index] ^= bit;
+      MPN_NORMALIZE (dp, dsize);
+      SIZ(d) = dsize;
+    }
+  else
+    {
+      mp_limb_t x = -dp[limb_index];
+      mp_size_t i;
+
+      /* non-zero limb below us means ones-complement */
+      for (i = limb_index-1; i >= 0; i--)
+	if (dp[i] != 0)
+	  {
+	    x--;  /* change twos comp to ones comp */
+	    break;
+	  }
+
+      if (x & bit)
+	{
+	  mp_limb_t  c;
+
+	  /* Clearing the bit increases the magitude. We might need a carry. */
+	  MPZ_REALLOC(d, dsize + 1);
+	  dp = LIMBS(d);
+
+	  __GMPN_ADD_1 (c, dp+limb_index, dp+limb_index,
+			dsize - limb_index, bit);
+	  dp[dsize] = c;
+	  dsize += c;
+	}
+      else
+	/* Setting the bit decreases the magnitude */
+	mpn_sub_1(dp+limb_index, dp+limb_index, dsize + limb_index, bit);
+
+      MPN_NORMALIZE (dp, dsize);
+      SIZ(d) = -dsize;
+    }
+}