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Pre-Depends, a More Demanding Depends
Pre-dependencies, which are listed in the Pre-Depends field in the package headers, complete the normal dependencies; their syntax is identical. A normal dependency indicates that the pack- age in question must be unpacked and configured before configuration of the package declaring the dependency. A pre-dependency stipulates that the package in question must be unpacked and configured before execution of the pre-installation script of the package declaring the pre- dependency, that is before its installation.
A pre-dependency is very demanding for apt because it adds a strict constraint on the ordering of the packages to install. As such, pre-dependencies are discouraged unless absolutely necessary. It is even recommended to consult other developers on [email protected] before adding a pre-dependency as it is generally possible to find another solution as a work-around.