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Using an undefined string or macro will cause it to be defined as empty.
Observed by Werner Lemberg on Nov 14, 2011
and rotting on my TODO list ever since.

.\" $OpenBSD: std.in,v 1.2 2017/07/04 14:53:27 schwarze Exp $
.TH STRING-UNDEF 1 "April 9, 2018"
.SH NAME
string-undef - expanding undefined strings
.SH DESCRIPTION
.SS User defined string
The sting "mys" is
.ie dmys defined
.el undefined
and remains
.ie dmys defined.
.el undefined.
.PP
Its value is "\*[mys]", and now it is
.ie dmys defined,
.el undefined,
and its value is still "\*[mys]".
.PP
.ds mys newval
After redefining it to "\*[mys]", it is of course still
.ie dmys defined.
.el undefined.
.PP
.rm mys
After removing the definition, it is now
.ie dmys defined.
.el undefined.
.SS User defined macro
The macro "mym" is
.ie dmym defined.
.el undefined.
.PP
It has no effect:
.mym
But now it is
.ie dmym defined.
.el undefined.
.PP
.de mym
neweffect
..
After defining it as:
.mym
it is of course still
.ie dmym defined.
.el undefined.
.PP
.rm mym
After removing the definition, it is now
.ie dmym defined.
.el undefined.
.SS Renamed macro
The standard .BR macro is
.ie dBR defined,
.el undefined,
and it
.BR works .
.PP
.rn BR newBR
After renaming it, the new name is
.ie dnewBR defined,
.el undefined,
and
.newBR works .
.SS Predefined string
A predefined string is
.ie dR defined
.el undefined
and has the value "\*R".