[RFA,4/4] Document prefix-define command and the use of . in command names.

diff --git a/gdb/NEWS b/gdb/NEWS
index 779fd91d3a..23772f41f6 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -27,6 +27,9 @@ 
   provide the exitcode or exit status of the shell commands launched by
   GDB commands such as "shell", "pipe" and "make".
 
+* User defined commands can now be defined using user defined prefix commands
+  Command names can now use the . character.
+
 * The RX port now supports XML target descriptions.
 
 * GDB now shows the Ada task names at more places, e.g. in task switching
@@ -69,6 +72,9 @@  pipe -d DELIM COMMAND DELIM SHELL_COMMAND
   With no COMMAND, repeat the last executed command
   and send its output to SHELL_COMMAND.
 
+prefix-define COMMAND
+  Define or mark a command as a user-defined prefix command.
+  
 with SETTING [VALUE] [-- COMMAND]
 w SETTING [VALUE] [-- COMMAND]
   Temporarily set SETTING, run COMMAND, and restore SETTING.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index f2713c0396..25ac7a03d1 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -26347,8 +26347,9 @@  end
 Define a command named @var{commandname}.  If there is already a command
 by that name, you are asked to confirm that you want to redefine it.
 The argument @var{commandname} may be a bare command name consisting of letters,
-numbers, dashes, and underscores.  It may also start with any predefined
-prefix command.  For example, @samp{define target my-target} creates
+numbers, dashes, dots, and underscores.  It may also start with any
+predefined or user-defined prefix command.
+For example, @samp{define target my-target} creates
 a user-defined @samp{target my-target} command.
 
 The definition of the command is made up of other @value{GDBN} command lines,
@@ -26369,6 +26370,37 @@  You may use the @code{document} command again to change the
 documentation of a command.  Redefining the command with @code{define}
 does not change the documentation.
 
+@kindex prefix-define
+@item prefix-define @var{commandname}
+Define or mark the command @var{commandname} as a user-defined prefix
+command.  Once marked, @var{commandname} can be used as prefix command
+by the  @code{define} command.
+Note that @code{prefix-define} can be used with a not yet defined
+@var{commandname}.  In such a case, @var{commandname} is defined as
+an empty user-defined command.
+
+Example:
+@example
+(gdb) prefix-define abc
+(gdb) prefix-define abc def
+(gdb) define abc def ghi
+Type commands for definition of "abc def ghi".
+End with a line saying just "end".
+>echo command ghi\n
+>end
+(gdb) define abc def
+Redefine command "def"? (y or n) y
+Type commands for definition of "abc def".
+End with a line saying just "end".
+>echo command def\n
+>end
+(gdb) abc def ghi
+command ghi
+(gdb) abc def
+command def
+(gdb)
+@end example
+
 @kindex dont-repeat
 @cindex don't repeat command
 @item dont-repeat