diff --git a/include/boost/contract/ext_/preprocessor/keyword/utility/is.hpp b/include/boost/contract/ext_/preprocessor/keyword/utility/is.hpp
index 3b267eb..286f85c 100644
--- a/include/boost/contract/ext_/preprocessor/keyword/utility/is.hpp
+++ b/include/boost/contract/ext_/preprocessor/keyword/utility/is.hpp
@@ -22,7 +22,8 @@
 // Precondition: A macro named `cat_to_comma_prefix ## token-to-check` must be
 //               #defined to expand to `,`.
 // Precondition: tokens must start with a token concatenable to a macro name
-//               (e.g., a literal or integral token) or with parenthesis.
+//               (e.g., a literal or integral token) or with parenthesis (i.e.,
+//               leading parenthesis are allowed).
 #define BOOST_CONTRACT_EXT_PP_KEYWORD_UTILITY_IS_FRONT( \
         cat_to_comma_prefix, tokens) \
     BOOST_PP_IIF(BOOST_CONTRACT_EXT_PP_HAS_PAREN(tokens), \
@@ -34,14 +35,12 @@
 // Precondition: A macro named `token-to-check ## cat_to_comma_postfix` must be
 //               #defined to expand to `,`.
 // Precondition: tokens must end with a token concatenable to a macro name
-//               (e.g., a literal or integral token) or with parenthesis.
+//               (e.g., a literal or integral token) and trailing parenthesis
+//               are NOT allowed.
 #define BOOST_CONTRACT_EXT_PP_KEYWORD_UTILITY_IS_BACK( \
         cat_to_comma_postfix, tokens) \
-    BOOST_PP_IIF(BOOST_CONTRACT_EXT_PP_HAS_PAREN(tokens), \
-        0 BOOST_PP_TUPLE_EAT(2) \
-    , \
-        BOOST_CONTRACT_EXT_PP_KEYWORD_UTILITY_IS_CHECK_ \
-    )(tokens, cat_to_comma_postfix)
+    BOOST_CONTRACT_EXT_PP_KEYWORD_UTILITY_IS_CHECK_( \
+            tokens, cat_to_comma_postfix)
 
 #endif // #include guard
 
diff --git a/include/boost/contract/ext_/preprocessor/utility/expand.hpp b/include/boost/contract/ext_/preprocessor/utility/expand.hpp
index 6652f4f..9587adb 100644
--- a/include/boost/contract/ext_/preprocessor/utility/expand.hpp
+++ b/include/boost/contract/ext_/preprocessor/utility/expand.hpp
@@ -4,6 +4,11 @@
 
 /* PUBLIC */
 
+// TODO: Is this /really/ needed? After all pp-traits are impl, try to remove
+// this and see if it's really needed.
+// TODO: Same thing for *any* use of BOOST_PP_EXPAND: make sure these are all
+// absolutely necessary!
+
 // Expand its argument once
 // NOTE: `BOOST_PP_EXPAND` expands it twice instead:
 //  EXPAND_ONCE(x)  --> x                   (x expanded once)
diff --git a/include/boost/contract/ext_/preprocessor/variadic/enum_to_seq.hpp b/include/boost/contract/ext_/preprocessor/variadic/enum_to_seq.hpp
new file mode 100644
index 0000000..a59ddd0
--- /dev/null
+++ b/include/boost/contract/ext_/preprocessor/variadic/enum_to_seq.hpp
@@ -0,0 +1,47 @@
+
+#ifndef BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ_HPP_
+#define BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ_HPP_
+
+#include <boost/contract/ext_/preprocessor/utility/nil.hpp>
+#include <boost/preprocessor/variadic/to_seq.hpp>
+#include <boost/preprocessor/seq/pop_back.hpp>
+#include <boost/preprocessor/seq/elem.hpp>
+#include <boost/preprocessor/seq/size.hpp>
+#include <boost/preprocessor/tuple/eat.hpp>
+#include <boost/preprocessor/arithmetic/dec.hpp>
+#include <boost/preprocessor/logical/compl.hpp>
+#include <boost/preprocessor/control/iif.hpp>
+
+/* PRIVATE */
+
+// Wrapped with parenthesis after removal of NIL so to make it a pp-seq elem.
+#define BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ_REMOVE_NIL_(back) \
+    (BOOST_CONTRACT_EXT_PP_NIL_REMOVE_BACK(back))
+
+#define BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ_BACK_(seq, back) \
+    BOOST_PP_SEQ_POP_BACK(seq) \
+    BOOST_PP_IIF(BOOST_CONTRACT_EXT_PP_IS_NIL_FRONT(back), \
+        BOOST_PP_TUPLE_EAT(1) \
+    , \
+        BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ_REMOVE_NIL_ \
+    )(back)
+
+#define BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ_(seq) \
+    BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ_BACK_(seq, \
+            BOOST_PP_SEQ_ELEM(BOOST_PP_DEC(BOOST_PP_SEQ_SIZE(seq)), seq))
+
+/* PUBLIC */
+
+// Expand EMPTY() to EMPTY(), otherwise variadic to seq allowing for an
+// optional trailing comma (like with C++ enumerations):
+//  ``              to ``
+//  `x, ..., y`     to (x)...(y)
+//  `x, ..., y,`    to (x)...(y) (handle optional trailing comma `,`)
+// Implementation: Tokens must be front-concatenable and cannot be specified as
+// NIL (because they are internally for being NIL).
+#define BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ(...) \
+    BOOST_CONTRACT_EXT_PP_VARIADIC_ENUM_TO_SEQ_( \
+            BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__ BOOST_PP_NIL))
+
+#endif // #include guard
+
diff --git a/include/boost/contract/ext_/preprocessor/variadic/to_seq.hpp b/include/boost/contract/ext_/preprocessor/variadic/to_seq.hpp
new file mode 100644
index 0000000..e30a5ab
--- /dev/null
+++ b/include/boost/contract/ext_/preprocessor/variadic/to_seq.hpp
@@ -0,0 +1,20 @@
+
+#ifndef BOOST_CONTRACT_EXT_PP_VARIADIC_TO_SEQ_HPP_
+#define BOOST_CONTRACT_EXT_PP_VARIADIC_TO_SEQ_HPP_
+
+#include <boost/contract/ext_/preprocessor/utility/empty.hpp>
+#include <boost/preprocessor/variadic/to_seq.hpp>
+#include <boost/preprocessor/control/iif.hpp>
+
+/* PUBLIC */
+
+// Expand EMPTY() to EMPTY(), otherwise expand variadic data to pp-seq.
+#define BOOST_CONTRACT_EXT_PP_VARIADIC_TO_SEQ(...) \
+    BOOST_PP_IIF(BOOST_CONTRACT_EXT_PP_IS_EMPTY(__VA_ARGS__), \
+        BOOST_PP_EMPTY \
+    , \
+        BOOST_PP_VARIADIC_TO_SEQ \
+    )(__VA_ARGS__)
+
+#endif // #include guard
+