use macro for /usr/local/plan9
diff --git a/man/man1/idiff.1 b/man/man1/idiff.1
index d497939..fda8458 100644
--- a/man/man1/idiff.1
+++ b/man/man1/idiff.1
@@ -61,7 +61,7 @@
 .SH FILES
 .B /tmp/idiff.*
 .SH SOURCE
-.B /usr/local/plan9/src/cmd/idiff.c
+.B \*9/src/cmd/idiff.c
 .SH "SEE ALSO
 .IR diff (1)
 .br