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Set a leading spaces trap. gtroff executes macro when it
encounters leading spaces in an input line; the implicit line break
that normally happens in this case is suppressed. A line consisting
of spaces only, however, is treated as an empty line, possibly subject
to an empty line macro set with the blm request.
Leading spaces are removed from the input line before calling the
leading spaces macro. The number of removed spaces is stored in
register lsn; the horizontal space that would be emitted if
there was no leading space macro is stored in register lss.
Note that lsn and lss are available even if no leading
space macro has been set.
The first thing a leading space macro sees is a token. However, some
escapes like \f or \m are handled on the fly (see
Gtroff Internals, for a complete list) without creating a token
at all. Consider that a line starts with two spaces followed by
\fIfoo. While skipping the spaces \fI is handled too so
that groff’s current font is properly set to ‘I’, but the leading
space macro only sees foo, without the preceding \fI.
If the macro should see the font escape you have to ‘protect’ it with
something that creates a token, for example with \&\fIfoo.