diff --git a/lib/io.cpp b/lib/io.cpp
index 3ea5068..336a191 100644
--- a/lib/io.cpp
+++ b/lib/io.cpp
@@ -4,86 +4,103 @@
 void
 writer::write_int (int x)
 {
-  uint8 buf[5];
-  unsigned n = 0;
-  
-  bool more = 1;
-  while (more)
+  if (raw)
+    write_raw (&x, sizeof x, 1);
+  else
     {
-      uint8 b = (uint8)(x & 0x7f);
-      x >>= 7;
+      uint8 buf[5];
+      unsigned n = 0;
       
-      if ((x == 0
-	   && ! (b & 0x40))
-	  || (x == -1
-	      && (b & 0x40) == 0x40))
-	more = 0;
-      else
-	b |= 0x80;
+      bool more = 1;
+      while (more)
+	{
+	  uint8 b = (uint8)(x & 0x7f);
+	  x >>= 7;
       
-      assert (n < 5);
-      buf[n] = b;
-      n ++;
+	  if ((x == 0
+	       && ! (b & 0x40))
+	      || (x == -1
+		  && (b & 0x40) == 0x40))
+	    more = 0;
+	  else
+	    b |= 0x80;
+      
+	  assert (n < 5);
+	  buf[n] = b;
+	  n ++;
+	}
+      
+      write_raw (buf, sizeof buf[0], n);
     }
-  
-  write_raw (buf, sizeof buf[0], n);
 }
 
 void
 writer::write_unsigned (unsigned x)
 {
-  uint8 buf[5];
-  unsigned n = 0;
-  
-  bool more = 1;
-  while (more)
+  if (raw)
+    write_raw (&x, sizeof x, 1);
+  else
     {
-      uint8 b = (uint8)(x & 0x7f);
-      x >>= 7;
-      
-      if ((x == 0
-	   && ! (b & 0x40)))
-	more = 0;
-      else
-	b |= 0x80;
-      
-      assert (n < 5);
-      buf[n] = b;
-      n ++;
-    }
+      uint8 buf[5];
+      unsigned n = 0;
   
-  write_raw (buf, sizeof buf[0], n);
+      bool more = 1;
+      while (more)
+	{
+	  uint8 b = (uint8)(x & 0x7f);
+	  x >>= 7;
+      
+	  if ((x == 0
+	       && ! (b & 0x40)))
+	    more = 0;
+	  else
+	    b |= 0x80;
+      
+	  assert (n < 5);
+	  buf[n] = b;
+	  n ++;
+	}
+  
+      write_raw (buf, sizeof buf[0], n);
+    }
 }
 
 void
 writer::write_uint64 (uint64 x)
 {
-  uint8 buf[10];
-  unsigned n = 0;
-  
-  bool more = 1;
-  while (more)
+  if (raw)
+    write_raw (&x, sizeof x, 1);
+  else
     {
-      uint8 b = (uint8)(x & 0x7f);
-      x >>= 7;
-      
-      if ((x == 0
-	   && ! (b & 0x40)))
-	more = 0;
-      else
-	b |= 0x80;
-
-      assert (n < 10);
-      buf[n] = b;
-      n ++;
-    }
+      uint8 buf[10];
+      unsigned n = 0;
   
-  write_raw (buf, sizeof buf[0], n);
+      bool more = 1;
+      while (more)
+	{
+	  uint8 b = (uint8)(x & 0x7f);
+	  x >>= 7;
+      
+	  if ((x == 0
+	       && ! (b & 0x40)))
+	    more = 0;
+	  else
+	    b |= 0x80;
+
+	  assert (n < 10);
+	  buf[n] = b;
+	  n ++;
+	}
+  
+      write_raw (buf, sizeof buf[0], n);
+    }
 }
 
 void
 writer::write_mpz (const mpz_t x)
 {
+  assert (!raw);
+  
   size_t count;
   void *buf = mpz_export (nullptr, &count, -1, 1, -1, 0, x);
   
@@ -97,20 +114,27 @@ int
 reader::read_int ()
 {
   int x = 0;
-  int shift = 0;
-  for (;;)
+  
+  if (raw)
+    read_raw (&x, sizeof x, 1);
+  else
     {
-      uint8 b = read_uint8 ();
-      x |= ((int)(b & 0x7f) << shift);
-      shift += 7;
-      if (! (b & 0x80))
+      int shift = 0;
+      for (;;)
 	{
-	  if (shift < int_bits
-	      && (b & 0x40))
-	    x = (x << (int_bits - shift)) >> (int_bits - shift);
-	  break;
+	  uint8 b = read_uint8 ();
+	  x |= ((int)(b & 0x7f) << shift);
+	  shift += 7;
+	  if (! (b & 0x80))
+	    {
+	      if (shift < int_bits
+		  && (b & 0x40))
+		x = (x << (int_bits - shift)) >> (int_bits - shift);
+	      break;
+	    }
 	}
     }
+  
   return x;
 }
 
@@ -118,15 +142,22 @@ unsigned
 reader::read_unsigned ()
 {
   unsigned x = 0;
-  unsigned shift = 0;
-  for (;;)
+  
+  if (raw)
+    read_raw (&x, sizeof x, 1);
+  else
     {
-      uint8 b = read_uint8 ();
-      x |= ((unsigned)(b & 0x7f) << shift);
-      shift += 7;
-      if (! (b & 0x80))
-	break;
+      unsigned shift = 0;
+      for (;;)
+	{
+	  uint8 b = read_uint8 ();
+	  x |= ((unsigned)(b & 0x7f) << shift);
+	  shift += 7;
+	  if (! (b & 0x80))
+	    break;
+	}
     }
+  
   return x;
 }
 
@@ -134,21 +165,30 @@ uint64
 reader::read_uint64 ()
 {
   uint64 x = 0;
-  uint64 shift = 0;
-  for (;;)
+
+  if (raw)
+    read_raw (&x, sizeof x, 1);
+  else
     {
-      uint8 b = read_uint8 ();
-      x |= ((uint64)(b & 0x7f) << shift);
-      shift += 7;
-      if (! (b & 0x80))
-	break;
+      uint64 shift = 0;
+      for (;;)
+	{
+	  uint8 b = read_uint8 ();
+	  x |= ((uint64)(b & 0x7f) << shift);
+	  shift += 7;
+	  if (! (b & 0x80))
+	    break;
+	}
     }
+  
   return x;
 }
 
 void
 reader::read_mpz (mpz_t x)
 {
+  assert (!raw);
+  
   unsigned count = read_unsigned ();
   void *p = malloc (count);
   if (!p)
@@ -212,6 +252,12 @@ file_writer::write_raw (const void *p, size_t itemsize, size_t nitems)
     }
 }
 
+void
+file_writer::write_mpz (const mpz_t x)
+{
+  mpz_out_raw (fp, x);
+}
+
 void
 file_reader::read_raw (void *p, size_t itemsize, size_t nitems)
 {
@@ -222,6 +268,12 @@ file_reader::read_raw (void *p, size_t itemsize, size_t nitems)
     }
 }
 
+void
+file_reader::read_mpz (mpz_t x)
+{
+  mpz_inp_raw (x, fp);
+}
+
 void
 gzfile_writer::write_raw (const void *p, size_t itemsize, size_t nitems)
 {
diff --git a/lib/io.h b/lib/io.h
index e08a243..80bf97d 100644
--- a/lib/io.h
+++ b/lib/io.h
@@ -1,9 +1,12 @@
 
 class writer
 {
+  // don't use leb128 enconding.  for backward compatibility.
+  bool raw;
+  
  public:
   writer (const writer &) = delete;
-  writer () = default;
+  writer (bool raw_ = false) : raw(raw_)  { }
   virtual ~writer () = default;
   
   writer &operator = (const writer &) = delete;
@@ -16,14 +19,16 @@ class writer
   void write_int (int x);
   void write_unsigned (unsigned x);
   void write_uint64 (uint64 x);
-  void write_mpz (const mpz_t x);
+  virtual void write_mpz (const mpz_t x);
 };
 
 class reader
 {
+  bool raw;
+  
  public:
   reader (const reader &) = delete;
-  reader () = default;
+  reader (bool raw_) : raw(raw_) { }
   virtual ~reader () = default;
   
   reader &operator = (const reader &) = delete;
@@ -54,7 +59,7 @@ class reader
   int read_int ();
   unsigned read_unsigned ();
   uint64 read_uint64 ();
-  void read_mpz (mpz_t x);
+  virtual void read_mpz (mpz_t x);
 };
 
 extern FILE *open_file (const std::string &file, const char *mode);
@@ -69,8 +74,9 @@ class file_writer : public writer
   FILE *fp;
   
  public:
-  file_writer (const std::string &file)
-    : fp(open_file (file, "w"))
+  file_writer (const std::string &file, bool raw = false)
+    : writer(raw),
+      fp(open_file (file, "w"))
   {
   }
   file_writer (const file_writer &) = delete;
@@ -79,6 +85,7 @@ class file_writer : public writer
   file_writer &operator = (const file_writer &) = delete;
   
   void write_raw (const void *p, size_t itemsize, size_t nitems);
+  void write_mpz (const mpz_t x);
 };
 
 class file_reader : public reader
@@ -87,8 +94,9 @@ class file_reader : public reader
   FILE *fp;
   
  public:
-  file_reader (const std::string &file)
-    : fp(open_file (file, "r"))
+  file_reader (const std::string &file, bool raw = false)
+    : reader(raw),
+      fp(open_file (file, "r"))
   {
   }
   file_reader (const file_reader &) = delete;
@@ -97,6 +105,7 @@ class file_reader : public reader
   file_reader &operator = (const file_reader &) = delete;
   
   void read_raw (void *p, size_t itemsize, size_t nitems);
+  void read_mpz (mpz_t x);
 };
 
 class gzfile_writer : public writer
@@ -106,7 +115,8 @@ class gzfile_writer : public writer
   
  public:
   gzfile_writer (const std::string &file)
-    : gzfp(open_gzfile (file, "w9"))
+    : writer(false),
+      gzfp(open_gzfile (file, "w9"))
   {
   }
   gzfile_writer (const gzfile_writer &) = delete;
@@ -124,7 +134,8 @@ class gzfile_reader : public reader
   
  public:
   gzfile_reader (const std::string &file)
-    : gzfp(open_gzfile (file, "r"))
+    : reader(false),
+      gzfp(open_gzfile (file, "r"))
   {
   }
   gzfile_reader (const gzfile_reader &) = delete;