* doc/Guidebook.mn: Make the Guidebook buildable from the top of the
source tree, not just inside the "doc" directory. Try to load its
"nh" macro package from the current working directory and from "doc".
* doc/tmac.nh: Allocate new register `nH` to the purpose of detecting
multiple loads, and skip file content if detected. This is the 1970s
nroff form of an "#include guard". groff's "an-ext.tmac" uses the
same technique for portability.
Also I removed a tab character. Per the groff Texinfo manual:
One possibly irritating idiosyncrasy is that tabs should not be
used to vertically align comments in the source document. Tab
characters are not treated as separators between a request name and
its first argument, nor between arguments.
Here's an example of how one groff macro package works around the
problem.
$ sed -n '402,406p' contrib/mm/m.tmac
.ds LetCN CONFIDENTIAL\" Confidential default
.ds LetSA To Whom It May Concern:\" Salutation default
.ds LetAT ATTENTION:\" Attention string
.ds LetSJ SUBJECT:\" Subject string
.ds LetRN In reference to:\" Reference string
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