+- implement parametric .de to support e.g.
+ /usr/xenocara/lib/libXxf86vm/manXF86VM.man
+ .de ZN
+ .ie t \fB\^\\$1\^\fR\\$2
+ .el \fI\^\\$1\^\fP\\$2
+ ..
+ .ZN -lXxf86vm
+
+- clean up escape sequence handling, creating three classes:
+ (1) fully implemented, or parsed and ignored without loss of content
+ (2) unimplemented, potentially causing loss of content
+ or serious mangling of formatting (e.g. \n) -> ERROR
+ see textproc/mgdiff(1) for nice examples
+ (3) undefined, just output the character -> perhaps WARNING
+