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The debian plugin calls which to determine whether aptitude & sudo are
installed.  However, if these are not installed then the error output
from which will be output.  Since this isn't really an error condition,
this commit updates the which calls to pipe stderr to stdout, thus
hiding the error messages.

Just an idea: change it to ap

Make the $apt_pref variable evaluatet at alias expansion by using single-quotes instad of double quotes