Some packages, have a
Provides: header. For example, the package
postfix on Debian Wheezy has
mail-transport-agent which doesn't exist physically is considered as a "virtual" package.
How can I know, on a Debian based system, if a given virtual package is "installed" (or "provided")? Can I list every "provided" virtual package?
Hint: to list every existing virtual package, installed or not, do:
aptitude search "~v"