= Alternation Syntax

Alternation syntax similar to that used in Unix shells may also be
used:

`%{%{Foo}:-bar}`

This code returns the value of `%{Foo}`, if it has a value.
Otherwise, it returns a literal string bar.

`%{%{Foo}:-%{Bar}}`

This code returns the value of `%{Foo}`, if it has a value.
Otherwise, it returns the expansion of `%{Bar}`.

These conditional expansions can be nested to almost any depth, such
as with `%{%{One}:-%{%{Two}:-%{Three}}}`.

.Examples
`%{%{Stripped-User-Name}:-%{User-Name}}` +
`%{%{Framed-IP-Address}:-<none>}`

// Copyright (C) 2020 Network RADIUS SAS.  Licenced under CC-by-NC 4.0.
// Development of this documentation was sponsored by Network RADIUS SAS.
