.ig
Define a string, then define an alias to it.
Redefining the original string should redefine the alias as well,
and vice-versa.
..
.ds xx This is a string.
.als yy xx
\*(xx
.br
\*(yy
.br
.ds xx This is a string, redefined the first time.
\*(xx
.br
\*(yy
.br
.ds yy This is a string, redefined the second time.
\*(xx
.br
\*(yy
.br
