tmac/tmac.an: define .MR in a groff compatible way

groff 1.23.0 added .MR to its -man macro package.  The NEWS file states
that the inclusion of the macro "was prompted by its introduction to
Plan 9 from User Space's troff in August 2020."  From d32deab it seems
that the name for Plan 9 from User Space's implementation was suggested
by groff maintainer G. Brandon Robinson.

Not sure if the intention was to make these definitions compatible, but
it would be nice if they were.

Currently, Plan 9 from User Space's .MR expects its second argument to
be parenthesized.  groff's .MR does not.  This results in extra
parentheses appearing in manual references when viewing Plan 9 from User
Space's manual pages on a system using groff.

1 files changed, 7 insertions, 7 deletions

diff --git a/man/man4/0intro.4 b/man/man4/0intro.4
index c80db208..2f3f8c2b 100644
--- a/man/man4/0intro.4
+++ b/man/man4/0intro.4
@@ -15,7 +15,7 @@ In Plan 9, the kernel mount device
 \fImnt\fR(3)
 acts as a client to the 9P servers mounted in the current name space,
 translating system calls such as
-.MR open (2)
+.MR open 2
 into 9P transactions such as
 .IR open (9p).
 The kernel also multiplexes the potentially many processes onto a single 9P conversation
@@ -30,17 +30,17 @@ On Unix, 9P clients do not access servers via the traditional
 file system call interface.  Only the Unix name space can be accessed
 that way.
 Instead, 9P clients use the
-.MR 9pclient (3)
+.MR 9pclient 3
 library to connect and interact directly with particular 9P servers.
 The
-.MR 9p (1)
+.MR 9p 1
 command-line client is useful for interactive use and in shell scripts.
 .PP
 To preserve the façade of a single 9P conversation with each server,
 9P servers invoke
-.MR 9pserve (4) ,
+.MR 9pserve 4 ,
 typically via
-.MR post9pservice (3) .
+.MR post9pservice 3 .
 .I 9pserve
 announces a 9P service at a particular
 network address and multiplexes the clients that connect to
@@ -58,7 +58,7 @@ Setting the
 .B $NAMESPACE
 environment variable overrides this default.
 The
-.MR namespace (1)
+.MR namespace 1
 command prints the current name space directory.
 .PP
 Occasionally it is useful to be able to connect the input or output
@@ -73,7 +73,7 @@ implementation of
 (see also
 .I fsopenfd
 in
-.MR 9pclient (3) )
+.MR 9pclient 3 )
 returns the read or write end of a pipe;
 a helper process transfers data between the other end of the pipe
 and the 9P server.