debconf directives
==================

cdebconf implements an extension to the Debconf specifications called
directives.  Directives are written in templates or substitutions enclosed into
"${!" and "}", for example "${!DIRECTIVE}".

 .  Directives are not expanded like normal variables and will be given back
/!\ as is by GET and METAGET commands.  This is the desired behaviour when,
~~~ for examples, one implements a dynamic Choices field for select questions
    with directives in SUBSTitutions.

Directives are expanded according to the result of the lookup_directive()
"method" implemented in the frontends when question_get_field() is called.
By default, they will be expanded to the empty string, removing them from the
returned value for unimplemented directives.