diff --git a/README.md b/README.md
index 94dc396..4694d78 100644
--- a/README.md
+++ b/README.md
@@ -1,21 +1,17 @@
-GM Serialization
-=====
+# GM Serialization
 
 Serialization helpers for the Gajumaru.
 
-Build
------
+## Build
 
     $ rebar3 compile
 
 
-Test
-----
+## Test
 
     $ rebar3 eunit
 
-Dynamic encoding
-----
+## Dynamic encoding
 
 The module `gmser_dyn` offers dynamic encoding support, encoding most 'regular'
 Erlang data types into an internal RLP representation.
@@ -34,6 +30,7 @@ how the type information is represented. The fully serialized form is
 produced by the `serialize` functions.
 
 The basic types supported by the encoder are:
+* `neg_integer()`     (`negint`, code: 247)
 * `non_neg_integer()` (`int`   , code: 248)
 * `binary()`          (`binary`, code: 249)
 * `boolean()`         (`bool`  , code: 250)
@@ -48,8 +45,8 @@ When encoding `map` types, the map elements are first sorted.
 When specifying a map type for template-driven encoding, use 
 the `#{items => [{Key, Value}]}` construct.
 
-Labels
-----
+
+## Labels
 
 Labels correspond to (existing) atoms in Erlang.
 Decoding of a label results in a call to `binary_to_existing_atom/2`, so will
@@ -63,8 +60,7 @@ Labels are encoded as `[<<255>>, << AtomToBinary/binary >>]`.
 If a cached label is used, the encoding becomes `[<<255>, [Ix]]`, where
 `Ix` is the integer-encoded index value of the cached label.
 
-Examples
-----
+## Examples
 
 Dynamically encoded objects have the basic structure `[<<0>>,V,Obj]`, where `V` is the
 integer-coded version, and `Obj` is the top-level encoding on the form `[Tag,Data]`.
@@ -88,8 +84,7 @@ Note that tuples and list are encoded the same way, except for the initial type
 Maps are encoded as `[<Map>, [KV1, KV2, ...]]`, where `[KV1, KV2, ...]` is the sorted
 list of key-value tuples from `map:to_list(Map)`, but with the `tuple` type tag omitted.
 
-Template-driven encoding
-----
+## Template-driven encoding
 
 Templates can be provided to the encoder by either naming an already registered
 type, or by passing a template directly. In both cases, the encoder will enforce
@@ -112,3 +107,23 @@ ET([{int,int}], [{1,2}]) -> [<<0>>,<<1>>,[<<251>>,[[[<<248>>,<<1>>],[<<248>>,<<2
 gmser_dyn:register_type(1000,lt2i,[{int,int}]).
 ET(lt2i, [{1,2}]) -> [<<0>>,<<1>>,[<<3,232>>,[[<<1>>,<<2>>]]]]
 ```
+
+### Alternative types
+
+The dynamic encoder supports two additions to the `gmserialization` template
+language: `any` and `#{alt => [AltTypes]}`.
+
+The `any` type doesn't have an associated code, but enforces dynamic encoding.
+
+The `#{alt => [Type]}` construct also enforces dynamic encoding, and will try
+to encode as each type in the list, in the specified order, until one matches.
+
+```erlang
+gmser_dyn:encode_typed(#{alt => [negint,int]}, 5) -> [<<0>>,<<1>>,[<<247>>,<<5>>]]
+gmser_dyn:encode_typed(#{alt => [negint,int]}, 5) -> [<<0>>,<<1>>,[<<248>>,<<5>>]]
+
+gmser_dyn:register_type(246, anyint, #{alt => [negint, int]})
+
+gmser_dyn:encode_typed(anyint,-5) -> [<<0>>,<<1>>,[<<246>>,[<<247>>,<<5>>]]]
+gmser_dyn:encode_typed(anyint,5)  -> [<<0>>,<<1>>,[<<246>>,[<<248>>,<<5>>]]]
+```
diff --git a/src/gmser_dyn.erl b/src/gmser_dyn.erl
index 52590f5..69ff56b 100644
--- a/src/gmser_dyn.erl
+++ b/src/gmser_dyn.erl
@@ -73,13 +73,12 @@ decode(Fields) ->
 decode(Fields0, Types) ->
     case decode_tag_and_vsn(Fields0) of
         {0, Vsn, Fields} ->
-            [Val] = decode_(Fields, Vsn, Types, []),
-            Val;
+            decode_(Fields, Vsn, Types);
         Other ->
             error({illegal_serialization, Other})
     end.
 
-decode_tag_and_vsn([TagBin, VsnBin | Fields]) ->
+decode_tag_and_vsn([TagBin, VsnBin, Fields]) ->
     {decode_basic(int, TagBin),
      decode_basic(int, VsnBin),
      Fields}.
@@ -87,7 +86,8 @@ decode_tag_and_vsn([TagBin, VsnBin | Fields]) ->
 dynamic_types() ->
     #{ vsn   => ?VSN
      , codes =>
-           #{ 248 => int
+           #{ 247 => negint
+            , 248 => int
             , 249 => binary
             , 250 => bool
             , 251 => list
@@ -96,7 +96,8 @@ dynamic_types() ->
             , 254 => id
             , 255 => label}
      , rev =>
-           #{ int    => 248
+           #{ negint => 247
+            , int    => 248
             , binary => 249
             , bool   => 250
             , list   => 251
@@ -107,7 +108,8 @@ dynamic_types() ->
      , labels => #{}
      , rev_labels => #{}
      , templates =>
-           #{ int    => int
+           #{ negint => negint
+            , int    => int
             , binary => binary
             , bool   => bool
             , list   => list
@@ -148,67 +150,70 @@ dyn_template_(F, Vsn) ->
 find_cached_label(Lbl, #{labels := Lbls}) ->
     maps:find(Lbl, Lbls).
 
-decode_(Fields, Vsn, Types, Acc) ->
-    {_Tag, Term, Rest} = decode_field_(Fields, Vsn, Types),
-    Acc1 = [Term | Acc],
-    case Rest of
-        [] ->
-            lists:reverse(Acc1);
-        _ ->
-            decode_(Rest, Vsn, Types, Acc1)
-    end.
-
-decode_field_([H|T], Vsn, Types) ->
-    {CodeBin, Field, Rest} =
-        case H of
-            [C, F] -> {C, F, T};
-            C when is_binary(C) -> {C, hd(T), tl(T)}
-        end,
+decode_([CodeBin, Flds], Vsn, Types) ->
     Code = decode_basic(int, CodeBin),
-    {Tag, Template} = template(Code, Vsn, Types),
-    %% [Fld|Rest] = Fields,
-    Val = decode_from_template(Template, Field, Vsn, Types),
-    {Tag, Val, Rest}.
+    {_Tag, Template} = template(Code, Vsn, Types),
+    decode_from_template(Template, Flds, Vsn, Types).
 
 encode_(Term, Vsn, Types) ->
-    encode_(Term, true, Vsn, Types).
+    encode_(Term, dyn(emit()), Vsn, Types).
 
-encode_(Term, Emit, Vsn, Types) ->
-    {Tag, Template} = auto_template(Term),
-    Enc = encode_from_template(Template, Term, Vsn, Types),
-    if Emit ->
-            [emit_code(Tag, Types), Enc];
-       true ->
-            Enc
+encode_(Term, E, Vsn, Types) ->
+    {_Tag, Template} = auto_template(Term),
+    encode_from_template(Template, Term, E, Vsn, Types).
+
+%% To control when to emit type codes:
+%% If the template is predefined, it's 'not dynamic' (nodyn(E)).
+%% If we are encoding against a type that's part of a predefined template,
+%% we typically don't emit the type code, except at the very top.
+%% So: emit type codes if the 'emit' bit is set, or if the 'dyn' bit is set.
+emit()    -> 2#01.
+dyn()     -> 2#10.
+emit(E)   -> E bor  2#01.
+noemit(E) -> E band 2#10.
+dyn(E)    -> E bor  2#10.
+nodyn(E)  -> E band 2#01. 
+
+encode_typed_(Type, Term, Vsn, #{codes := Codes, rev := Rev} = Types) ->
+    case (is_map_key(Type, Codes) orelse is_map_key(Type, Rev)) of
+        true ->
+            encode_typed_(Type, Term, nodyn(emit()), Vsn, Types);
+        false ->
+            encode_maybe_template(Type, Term, Vsn, Types)
     end.
 
-encode_typed_(Type, Term, Vsn, Types) ->
-    encode_typed_(Type, Term, true, Vsn, Types).
-
 encode_typed_(any, Term, _, Vsn, Types) ->
-    encode_(Term, true, Vsn, Types);
-encode_typed_(Code, Term, Emit, Vsn, #{codes := Codes} = Types) when is_map_key(Code, Codes) ->
+    encode_(Term, dyn(emit()), Vsn, Types);
+encode_typed_(Code, Term, E, Vsn, #{codes := Codes} = Types) when is_map_key(Code, Codes) ->
     {_Tag, Template} = template(Code, Vsn, Types),
-    maybe_emit(Emit, Code, encode_from_template(Template, Term, false, Vsn, Types));
-encode_typed_(Tag, Term, Emit, Vsn, #{templates := Ts, rev := Rev} = Types)
+    [encode_basic(int,Code),
+     encode_from_template(Template, Term, noemit(nodyn(E)), Vsn, Types)];
+encode_typed_(Tag, Term, E, Vsn, #{templates := Ts, rev := Rev} = Types)
   when is_map_key(Tag, Ts) ->
     Template = dyn_template_(maps:get(Tag, Ts), Vsn),
     Code = maps:get(Tag, Rev),
-    maybe_emit(Emit, Code, encode_from_template(Template, Term, false, Vsn, Types));
+    [encode_basic(int,Code),
+     encode_from_template(Template, Term, noemit(nodyn(E)), Vsn, Types)];
 encode_typed_(MaybeTemplate, Term, _, Vsn, Types) ->
     encode_maybe_template(MaybeTemplate, Term, Vsn, Types).
 
-maybe_emit(true, Code, Enc) ->
+maybe_emit(E, Code, Enc) when E > 0 ->
     [encode_basic(int, Code), Enc];
-maybe_emit(false, _, Enc) ->
+maybe_emit(0, _, Enc) ->
     Enc.
 
+encode_maybe_template(#{items := _} = Type, Term, Vsn, Types) ->
+    case is_map(Term) of
+        true ->
+            encode_from_template(Type, Term, emit(dyn()), Vsn, Types);
+        false ->
+            error({invalid, Type, Term})
+    end;
+encode_maybe_template(#{alt := _} = Type, Term, Vsn, Types) ->
+    encode_from_template(Type, Term, Vsn, emit(dyn()), Types);
 encode_maybe_template(Pat, Term, Vsn, Types) when is_list(Pat);
-                                                  is_tuple(Pat);
-                                                  is_map(Pat) ->
-    {Tag, _} = auto_template(Pat),
-    [emit_code(Tag, Types),
-     encode_from_template(Pat, Term, true, Vsn, Types)];
+                                                  is_tuple(Pat) ->
+    encode_from_template(Pat, Term, emit(dyn()), Vsn, Types);
 encode_maybe_template(Other, Term, _Vsn, _Types) ->
     error({illegal_template, Other, Term}).
 
@@ -233,19 +238,32 @@ auto_template(T) ->
        is_atom(T)    -> {label, label};   % binary_to_existing_atom()
        is_integer(T),
        T >= 0        -> {int, int};
+       is_integer(T),
+       T < 0         -> {negint, negint};
        true ->
-            error(invalid_type)
+            error({invalid_type, T})
     end.
 
-decode_from_template(list, Fld, Vsn, Types) ->
-    decode_(Fld, Vsn, Types, []);
+decode_from_template(any, Fld, Vsn, Types) ->
+    decode_(Fld, Vsn, Types);
+decode_from_template(#{items := Items}, Fld, Vsn, Types) when is_list(Fld) ->
+    Zipped = lists:zip(Items, Fld),
+    lists:foldl(
+      fun({{K, Type}, V}, Map) ->
+              maps:is_key(K, Map) andalso error(badarg, duplicate_field),
+              Map#{K => decode_from_template({any,Type}, V, Vsn, Types)}
+      end, #{}, Zipped);
+decode_from_template(#{alt := Alts} = T, Fld, Vsn, Types) when is_list(Alts) ->
+    decode_alt(Alts, Fld, T, Vsn, Types);
+decode_from_template(list, Flds, Vsn, Types) ->
+    [decode_(F, Vsn, Types) || F <- Flds];
 decode_from_template(map, Fld, Vsn, Types) ->
     TupleFields = [F || F <- Fld],
-    Items = [decode_from_template(tuple, T, Vsn, Types)
+    Items = [decode_from_template({any,any}, T, Vsn, Types)
              || T <- TupleFields],
     maps:from_list(Items);
 decode_from_template(tuple, Fld, Vsn, Types) ->
-    Items = decode_(Fld, Vsn, Types, []),
+    Items = [decode_(F, Vsn, Types) || F <- Fld],
     list_to_tuple(Items);
 decode_from_template([Type], Fields, Vsn, Types) ->
     [decode_from_template(Type, F, Vsn, Types)
@@ -260,72 +278,109 @@ decode_from_template(label, [C], _, #{rev_labels := RLbls}) ->
     Code = decode_basic(int, C),
     maps:get(Code, RLbls);
 decode_from_template(Type, Fld, _, Types) when Type == int
+                                               ; Type == negint
                                                ; Type == binary
                                                ; Type == bool
                                                ; Type == id
                                                ; Type == label ->
     decode_basic(Type, Fld, Types).    
 
-encode_from_template(Type, V, Vsn, Types) ->
-    encode_from_template(Type, V, true, Vsn, Types).
-
-encode_from_template(any, V, _, Vsn, Types) ->
-    encode_(V, true, Vsn, Types);
-encode_from_template(list, L, _, Vsn, Types) when is_list(L) ->
+encode_from_template(any, V, _E, Vsn, Types) ->
+    encode_(V, dyn(emit()), Vsn, Types);
+encode_from_template(list, L, E, Vsn, Types) when is_list(L) ->
     assert_type(is_list(L), list, L),
-    [encode_(V, Vsn, Types) || V <- L];
-encode_from_template(map, M, _, Vsn, Types) ->
+    emit(E, list, Types,
+         [encode_(V, Vsn, Types) || V <- L]);
+encode_from_template(#{items := Items}, M, E, Vsn, Types) ->
     assert_type(is_map(M), map, M),
-    [encode_({K,V}, false, Vsn, Types)
-     || {K, V} <- lists:sort(maps:to_list(M))];
-encode_from_template(tuple, T, Emit, Vsn, Types) ->
+    Emit = noemit(E),
+    emit(E, map, Types,
+         lists:map(
+           fun({K, Type}) ->
+                   V = maps:get(K, M),
+                   [encode_from_template(any, K, Emit, Vsn, Types),
+                    encode_from_template(Type, V, Emit, Vsn, Types)]
+           end, Items));
+encode_from_template(#{alt := Alts} = T, Term, E, Vsn, Types) when is_list(Alts) ->
+    encode_alt(Alts, Term, T, E, Vsn, Types);
+encode_from_template(map, M, E, Vsn, Types) ->
+    assert_type(is_map(M), map, M),
+    Emit = emit(E),
+    emit(E, map, Types,
+         [[encode_from_template(any, K, Emit, Vsn, Types),
+           encode_from_template(any, V, Emit, Vsn, Types)]
+          || {K, V} <- lists:sort(maps:to_list(M))]);
+encode_from_template(tuple, T, E, Vsn, Types) ->
     assert_type(is_tuple(T), tuple, T),
-    [encode_(V, Emit, Vsn, Types) || V <- tuple_to_list(T)];
-encode_from_template(T, V, Emit, Vsn, Types) when is_tuple(T) ->
+    emit(E, tuple, Types,
+         [encode_(V, noemit(E), Vsn, Types) || V <- tuple_to_list(T)]);
+encode_from_template(T, V, E, Vsn, Types) when is_tuple(T) ->
     assert_type(is_tuple(V), T, V),
     assert_type(tuple_size(T) =:= tuple_size(V), T, V),
     Zipped = lists:zip(tuple_to_list(T), tuple_to_list(V)),
-    [encode_from_template(T1, V1, Emit, Vsn, Types) || {T1, V1} <- Zipped];
-encode_from_template([Type] = T, List, Emit, Vsn, Types) ->
+    emit(E, tuple, Types,
+         [encode_from_template(T1, V1, noemit(E), Vsn, Types) || {T1, V1} <- Zipped]);
+encode_from_template([Type] = T, List, E, Vsn, Types) ->
     assert_type(is_list(List), T, List),
-    [encode_from_template(Type, V, Emit, Vsn, Types) || V <- List];
-encode_from_template(Type, List, Emit, Vsn, Types) when is_list(Type), is_list(List) ->
-    encode_fields(Type, List, Emit, Vsn, Types);
-encode_from_template(label, V, Emit, _, Types) ->
+    emit(E, list, Types,
+         [encode_from_template(Type, V, noemit(E), Vsn, Types) || V <- List]);
+encode_from_template(Type, List, E, Vsn, Types) when is_list(Type), is_list(List) ->
+    encode_fields(Type, List, E, Vsn, Types);
+encode_from_template(label, V, E, _, Types) ->
     assert_type(is_atom(V), label, V),
-    case find_cached_label(V, Types) of
-        error ->
-            encode_basic(label, V, Emit, Types);
-        {ok, Code} when is_integer(Code) ->
-            [encode_basic(int, Code)]
-    end;
-encode_from_template(Type, V, Emit, _, Types) when Type == id
-                                                   ; Type == binary
-                                                   ; Type == bool
-                                                   ; Type == int
-                                                   ; Type == label ->
-    encode_basic(Type, V, Emit, Types);
-encode_from_template(Type, V, Emit, Vsn, Types) ->
-    encode_typed_(Type, V, Emit, Vsn, Types).
-    %% error({illegal, Type, V}).
+    emit(E, label, Types,
+         case find_cached_label(V, Types) of
+             error ->
+                 encode_basic(label, V, E, Types);
+             {ok, Code} when is_integer(Code) ->
+                 [encode_basic(int, Code)]
+         end);
+encode_from_template(Type, V, E, _, Types) when Type == id
+                                                ; Type == binary
+                                                ; Type == bool
+                                                ; Type == int
+                                                ; Type == negint
+                                                ; Type == label ->
+    encode_basic(Type, V, E, Types);
+encode_from_template(Type, V, E, Vsn, Types) ->
+    encode_typed_(Type, V, E, Vsn, Types).
 
 assert_type(true, _, _) -> ok;
 assert_type(_, Type, V) -> error({illegal, Type, V}).
 
+decode_alt([A|Alts], Fld, T, Vsn, Types) ->
+    try decode_from_template(A, Fld, Vsn, Types)
+    catch error:_ ->
+            decode_alt(Alts, Fld, T, Vsn, Types)
+    end;
+decode_alt([], Fld, T, _, _) ->
+    error({illegal, T, Fld}).
+
+encode_alt(Alts, Term, T, E, Vsn, Types) ->
+    %% Since we don't know which type may match, treat as dynamic.
+    encode_alt_(Alts, Term, T, dyn(E), Vsn, Types).
+
+encode_alt_([A|Alts], Term, T, E, Vsn, Types) ->
+    try encode_from_template(A, Term, E, Vsn, Types)
+    catch error:_ ->
+            encode_alt_(Alts, Term, T, E, Vsn, Types)
+    end;
+encode_alt_([], Term, T, _, _, _) ->
+    error({illegal, T, Term}).
 
 %% Basically, dynamically encoding a statically defined object
 encode_fields([{Field, Type}|TypesLeft],
-              [{Field,  Val}|FieldsLeft], Emit, Vsn, Types) ->
-    [ encode_from_template(Type, Val, Emit, Vsn, Types)
-    | encode_fields(TypesLeft, FieldsLeft, Emit, Vsn, Types)];
+              [{Field,  Val}|FieldsLeft], E, Vsn, Types) ->
+    [ encode_from_template(Type, Val, E, Vsn, Types)
+    | encode_fields(TypesLeft, FieldsLeft, E, Vsn, Types)];
 encode_fields([{_Field, _Type} = FT|_TypesLeft],
-              [Val                 |_FieldsLeft], _Emit, _Vsn, _Types) ->
+              [Val                 |_FieldsLeft], _E, _Vsn, _Types) ->
     error({illegal_field, FT, Val});
 encode_fields([Type|TypesLeft],
-              [Val |FieldsLeft], Emit, Vsn, Types) when is_atom(Type) ->
+              [Val |FieldsLeft], E, Vsn, Types) when is_atom(Type) ->
     %% Not sure about this ...
-    [ encode_from_template(Type, Val, Emit, Vsn, Types)
-    | encode_fields(TypesLeft, FieldsLeft, Emit, Vsn, Types)];
+    [ encode_from_template(Type, Val, E, Vsn, Types)
+    | encode_fields(TypesLeft, FieldsLeft, E, Vsn, Types)];
 encode_fields([], [], _, _, _) ->
     [].
 
@@ -337,6 +392,12 @@ decode_fields([{Tag, Type}|TypesLeft],
 decode_fields([], [], _, _) ->
     [].
 
+
+emit(E, Tag, Types, Enc) when E > 0 ->
+    [emit_code(Tag, Types), Enc];
+emit(0, _, _, Enc) ->
+    Enc.
+
 emit_code(Tag, #{rev := Tags}) ->
     encode_basic(int, maps:get(Tag, Tags)).
 
@@ -352,12 +413,17 @@ decode_basic(Type, V, _) ->
 
 decode_basic(label, Fld) ->
     binary_to_existing_atom(decode_basic(binary, Fld), utf8);
+decode_basic(negint, Fld) ->
+    I = gmserialization:decode_field(int, Fld),
+    -I;
 decode_basic(Type, Fld) ->
     gmserialization:decode_field(Type, Fld).
 
-encode_basic(Tag, V, true, Types) ->
+encode_basic(negint, I, _, Types) when is_integer(I), I < 0 ->
+    [emit_code(negint, Types), gmserialization:encode_field(int, -I)];
+encode_basic(Tag, V, E, Types) when E > 0 ->
     [emit_code(Tag, Types), encode_basic(Tag, V)];
-encode_basic(Tag, V, false, _) ->
+encode_basic(Tag, V, 0, _) ->
     encode_basic(Tag, V).
 
 encode_basic(label, A) when is_atom(A) ->
@@ -536,6 +602,7 @@ round_trip_test_() ->
 
 t_sample_types() ->
     [ 5
+    , -5
     , <<"a">>
     , [1,2,3]
     , {<<"a">>,1}
@@ -560,10 +627,20 @@ user_types_test_() ->
      , ?_test(t_reg_label_cache2())
      ]}.
 
+dynamic_types_test_() ->
+    [ ?_test(revert_to_default_types())
+    , ?_test(t_typed_map())
+    , ?_test(t_alts())
+    ].
+
 t_round_trip(T) ->
     ?debugVal(T),
     ?assertMatch({T, T}, {T, decode(encode(T))}).
 
+t_round_trip_typed(Type, T) ->
+    ?debugVal(T),
+    ?assertMatch({T, T}, {T, decode(encode_typed(Type, T))}).
+
 t_reg_typed_tuple() ->
     Type = {int, int, int},
     MyTypes = #{ codes => #{ 1001 => int_tup3 }
@@ -645,7 +722,16 @@ t_reg_label_cache2() ->
     Tup = {'1', '1'},
     Enc = gmser_dyn:encode_typed(lbl_tup2, Tup),
     [<<0>>,<<1>>,[<<3,235>>,[[<<49>>],[<<49>>]]]] = Enc,
-    Tup = gmser_dyn:decode(Enc).
+    _Tup = gmser_dyn:decode(Enc).
 
+t_typed_map() ->
+    Term = #{a => 13, {key,1} => [a]},
+    Enc = encode_typed(#{items => [{a,int},{{key,1},[label]}]}, Term),
+    ?assertEqual(Term, decode(Enc)).
+
+t_alts() ->
+    t_round_trip_typed(#{alt => [negint, int]}, -4),
+    t_round_trip_typed(#{alt => [negint, int]}, 4),
+    ok.
 
 -endif.