Add extra rule to prevent ambiguou extension name with version

- The second letter from the bottom cannot be a numeric if the
  last letter is ``p''

- For example, Zfoo1p0 is ambiguou without this extra rule:
  Zfoo1p extension with version 0 or Zfoo extension with version 1.0.

src/naming.tex

@@ -77,10 +77,11 @@ \section{Underscores}
 \section{Additional Standard Extension Names}
 
 Standard extensions can also be named using a single ``Z'' followed by an
-alphanumeric name, the name must end with an alphabetical, and an optional
-version number.  For example, ``Zifencei'' names the instruction-fetch fence
-extension described in Chapter~\ref{chap:zifencei}; ``Zifencei2'' and
-``Zifencei2p0'' name version 2.0 of same.
+alphanumeric name, the name must end with an alphabetical, an optional
+version number, and the second letter from the bottom cannot be a numeric if the
+last letter is ``p''.  For example, ``Zifencei'' names the instruction-fetch
+fence extension described in Chapter~\ref{chap:zifencei}; ``Zifencei2'' and
+``Zifencei2p0'' name version 2.0 are the same.
 
 The first letter following the ``Z'' conventionally indicates the most closely
 related alphabetical extension category, IMAFDQLCBKJTPV.  For the ``Zam''
@@ -97,7 +98,8 @@ \section{Supervisor-level Instruction-Set Extensions}
 
 Standard supervisor-level instruction-set extensions are defined in Volume II,
 but are named using ``S'' as a prefix, followed by an alphanumeric name, the
-name must end with an alphabetical, and an optional version number.
+name must end with an alphabetical, an optional version number, and the second
+letter from the bottom cannot be a numeric if the last letter is ``p''.
 Supervisor-level extensions must be separated from
 other multi-letter extensions by an underscore.
 
@@ -127,8 +129,9 @@ \section{Machine-level Instruction-Set Extensions}
 \section{Non-Standard Extension Names}
 
 Non-standard extensions are named using a single ``X'' followed by an
-alphanumeric name, the name must end with an alphabetical, and an optional
-version number.
+alphanumeric name, the name must end with an alphabetical, an optional
+version number, and the second letter from the bottom cannot be a numeric if the
+last letter is ``p''.
 For example, ``Xhwacha'' names the Hwacha vector-fetch ISA extension;
 ``Xhwacha2'' and ``Xhwacha2p0'' name version 2.0 of same.