5.6.2 Invoking Requests

A control character is optionally followed by tabs and/or spaces and then an identifier naming a request or macro. The invocation of an unrecognized request is interpreted as a macro call. Defining a macro with the same name as a request replaces the request. Deleting a request name with the rm request makes it unavailable. The als request can alias requests, permitting them to be wrapped or non-destructively replaced.⁷⁶

There is no inherent limit on argument length or quantity. Most requests take one or more arguments, and ignore any they do not expect. A request that is not supplied a mandatory argument does nothing.⁷⁷ Spaces or tabs separate a request’s name from its first argument, but only spaces can separate an argument from its successor.⁷⁸ Only one space between arguments is necessary; the formatter ignores any excess. GNU troff does not interpret tabs as argument separators.⁷⁹

Generally, a space within a request argument is not relevant, not meaningful, or is supported by bespoke provisions, as with the tl request’s delimiters.⁸⁰ Some requests, like ds, interpret the remainder of the control line as a single argument.⁸¹

Commonly, authors use spaces and/or tabs after a control character to indent the source of documents or macro files. The formatter ignores spaces and tabs immediately after a control character, except that the presence of a tab prevents recognition of the end of a macro definition.

.de center
.  if \\n[.br] \
.    br
.  ce \\$1
..
.
.
.de right-align
.→if \\n[.br] \
.→→br
.→rj \\$1
..

If you assign an empty blank line trap, you can separate macro definitions (or any input lines) with blank lines.⁸²

.de do-nothing
..
.blm do-nothing  \" activate blank line trap

.de center
.  if \\n[.br] \
.    br
.  ce \\$1
..


.de right-align
.→if \\n[.br] \
.→→br
.→rj \\$1
..

.blm             \" deactivate blank line trap