diff --git a/implementations/javascript/src/codec.js b/implementations/javascript/src/codec.js
index 25595c1..61482e0 100644
--- a/implementations/javascript/src/codec.js
+++ b/implementations/javascript/src/codec.js
@@ -13,6 +13,7 @@ class EncodeError extends Error {
     this.irritant = irritant;
   }
 }
+class ShortPacket extends DecodeError {}
 
 class Decoder {
   constructor(packet, options) {
@@ -22,22 +23,18 @@ class Decoder {
     this.shortForms = options.shortForms || {};
   }
 
-  peekbyte() {
-    if (this.index >= this.packet.length) throw new DecodeError("Short packet");
-    // ^ NOTE: greater-than-or-equal-to, not greater-than.
-    return this.packet[this.index];
-  }
-
-  advance(count) {
-    const start = this.index;
-    this.index += (count === void 0 ? 1 : count);
-    return start;
-  }
-
   nextbyte() {
-    const val = this.peekbyte();
-    this.advance();
-    return val;
+    if (this.index >= this.packet.length) throw new ShortPacket("Short packet");
+    // ^ NOTE: greater-than-or-equal-to, not greater-than.
+    return this.packet[this.index++];
+  }
+
+  nextbytes(n) {
+    const start = this.index;
+    this.index += n;
+    if (this.index > this.packet.length) throw new ShortPacket("Short packet");
+    // ^ NOTE: greater-than, not greater-than-or-equal-to.
+    return new DataView(this.packet.buffer, this.packet.byteOffset + start, n);
   }
 
   wirelength(arg) {
@@ -52,36 +49,25 @@ class Decoder {
     return (this.varint() << 7) + (v - 128);
   }
 
-  nextbytes(n) {
-    const start = this.advance(n);
-    if (this.index > this.packet.length) throw new DecodeError("Short packet");
-    // ^ NOTE: greater-than, not greater-than-or-equal-to.
-    return new DataView(this.packet.buffer, this.packet.byteOffset + start, n);
-  }
-
   nextvalues(n) {
     const result = [];
     for (let i = 0; i < n; i++) result.push(this.next());
     return result;
   }
 
-  peekop() {
-    const b = this.peekbyte();
+  nextop() {
+    const b = this.nextbyte();
     const major = b >> 6;
     const minor = (b >> 4) & 3;
     const arg = b & 15;
     return [major, minor, arg];
   }
 
-  nextop() {
-    const op = this.peekop();
-    this.advance();
-    return op;
-  }
-
   peekend(arg) {
-    const [a,i,r] = this.peekop();
-    return (a === 0) && (i === 3) && (r === arg);
+    const [a,i,r] = this.nextop();
+    const result = (a === 0) && (i === 3) && (r === arg);
+    if (!result) this.index--;
+    return result;
   }
 
   binarystream(arg, minor) {
@@ -335,6 +321,7 @@ class Encoder {
 Object.assign(module.exports, {
   DecodeError,
   EncodeError,
+  ShortPacket,
   Decoder,
   Encoder,
 });
diff --git a/implementations/javascript/test/samples.txt b/implementations/javascript/test/samples.txt
index 7721127..397b7ea 100644
--- a/implementations/javascript/test/samples.txt
+++ b/implementations/javascript/test/samples.txt
@@ -12,6 +12,7 @@
 26626865626c6c6060616f36
 27626865626c6c6060616f37
 2c111213143c
+2c2563616263352563646566353c
 2cc2516111c2516212c25163133c
 410d
 417f
diff --git a/implementations/javascript/test/test-codec.js b/implementations/javascript/test/test-codec.js
index 0d5f93c..2102690 100644
--- a/implementations/javascript/test/test-codec.js
+++ b/implementations/javascript/test/test-codec.js
@@ -71,6 +71,7 @@ describe('hex samples', () => {
     { expected: Bytes.from("hello"), encodesTo: '6568656c6c6f', },
     { expected: Symbol.for("hello"), encodesTo: '7568656c6c6f', },
     { expected: Immutable.Seq([1, 2, 3, 4]), },
+    { expected: Preserves.fromJS(["abc", "def"]), encodesTo: 'c25361626353646566' },
     { expected: Preserves.fromJS([["a", 1], ["b", 2], ["c", 3]]),
       encodesTo: 'c3c2516111c2516212c2516313', },
     { expected: 13, },
diff --git a/implementations/python/preserves/__init__.py b/implementations/python/preserves/__init__.py
index 2d44921..22d6568 100644
--- a/implementations/python/preserves/__init__.py
+++ b/implementations/python/preserves/__init__.py
@@ -1,6 +1,6 @@
 from .preserves import Float, Symbol, Record, ImmutableDict
 
-from .preserves import DecodeError, EncodeError
+from .preserves import DecodeError, EncodeError, ShortPacket
 
 from .preserves import Decoder, Encoder
 
diff --git a/implementations/python/preserves/preserves.py b/implementations/python/preserves/preserves.py
index 4001898..a3cc8e1 100644
--- a/implementations/python/preserves/preserves.py
+++ b/implementations/python/preserves/preserves.py
@@ -119,6 +119,7 @@ def dict_kvs(d):
 
 class DecodeError(ValueError): pass
 class EncodeError(ValueError): pass
+class ShortPacket(DecodeError): pass
 
 class Codec(object):
     def __init__(self):
@@ -183,21 +184,19 @@ class Decoder(Codec):
         self.packet = packet
         self.index = 0
 
-    def peekbyte(self):
-        if self.index < len(self.packet):
-            return _ord(self.packet[self.index])
-        else:
-            raise DecodeError('Short packet')
-
-    def advance(self, count=1):
-        start = self.index
-        self.index = self.index + count
-        return start
-
     def nextbyte(self):
-        val = self.peekbyte()
-        self.advance()
-        return val
+        if self.index >= len(self.packet):
+            raise ShortPacket('Short packet')
+        self.index = self.index + 1
+        return _ord(self.packet[self.index - 1])
+
+    def nextbytes(self, n):
+        start = self.index
+        end = start + n
+        if end > len(self.packet):
+            raise ShortPacket('Short packet')
+        self.index = end
+        return self.packet[start : end]
 
     def wirelength(self, arg):
         if arg < 15:
@@ -211,30 +210,24 @@ class Decoder(Codec):
         else:
             return self.varint() * 128 + (v - 128)
 
-    def nextbytes(self, n):
-        start = self.advance(n)
-        return self.packet[start : self.index]
-
     def nextvalues(self, n):
         result = []
         for i in range(n):
             result.append(self.next())
         return result
 
-    def peekop(self):
-        b = self.peekbyte()
+    def nextop(self):
+        b = self.nextbyte()
         major = b >> 6
         minor = (b >> 4) & 3
         arg = b & 15
         return (major, minor, arg)
 
-    def nextop(self):
-        op = self.peekop()
-        self.advance()
-        return op
-
     def peekend(self, arg):
-        return self.peekop() == (0, 3, arg)
+        matched = (self.nextop() == (0, 3, arg))
+        if not matched:
+            self.index = self.index - 1
+        return matched
 
     def binarystream(self, arg, minor):
         result = []
diff --git a/implementations/python/preserves/test_preserves.py b/implementations/python/preserves/test_preserves.py
index c0e6767..0131b4d 100644
--- a/implementations/python/preserves/test_preserves.py
+++ b/implementations/python/preserves/test_preserves.py
@@ -199,3 +199,8 @@ class CodecTests(unittest.TestCase):
         self._roundtrip((False,) * 15, _buf(0xCF, 0x0F, b'\x00' * 15))
         self._roundtrip((False,) * 100, _buf(0xCF, 0x64, b'\x00' * 100))
         self._roundtrip((False,) * 200, _buf(0xCF, 0xC8, 0x01, b'\x00' * 200))
+
+    def test_format_c_twice(self):
+        self._roundtrip(SequenceStream([StringStream([b'abc']), StringStream([b'def'])]),
+                        _buf(0x2C, 0x25, 0x63, 'abc', 0x35, 0x25, 0x63, 'def', 0x35, 0x3C),
+                        back=(u'abc', u'def'))