aptitude::Delete-Unused behaves incorrectly

[任晓磊]

Package: aptitude
Version: 0.4.9-2ubuntu5
Severity: normal

when I execute "aptitude install trac", libapr1 is installed, too ,for
dependencies.
then, I execute "aptitude install apache2", apache2-mpm-worker is
installed for dependencies.

later, I "aptitude purge apache2", it prompted me that only apache2
package will be purged.

I guess:
trac depend on subverion, subversion depend on libapr1.
apache2 depend on apache2-mpm-worker, apache2-mpm-worker depend on libapr1.

When I purge apache2, libapr1 is still depended on by trac/subverion.
Aptitude thinks that apache2-mpm-worker (the package associated to
libapr1) shouldn't be uninstall.
This is wrong.

-- System Information:
Debian Release: lenny/sid
  APT prefers hardy-updates
  APT policy: (500, 'hardy-updates'), (500, 'hardy-security'), (500, 'hardy')
Architecture: i386 (i686)

Kernel: Linux 2.6.24-16-generic (SMP w/1 CPU core)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages aptitude depends on:
ii  apt [libapt-pkg-li 0.7.9ubuntu16         Advanced front-end for dpkg
ii  libc6              2.7-10ubuntu3         GNU C Library: Shared libraries
ii  libcwidget3        0.5.8-1ubuntu1        high-level terminal interface libr
ii  libgcc1            1:4.2.3-2ubuntu7      GCC support library
ii  libncursesw5       5.6+20071124-1ubuntu2 Shared libraries for terminal hand
ii  libsigc++-2.0-0c2a 2.0.17-2ubuntu3       type-safe Signal Framework for C++
ii  libstdc++6         4.2.3-2ubuntu7        The GNU Standard C++ Library v3

Versions of packages aptitude recommends:
pn  aptitude-doc-en | aptitude-do <none>     (no description available)
pn  libparse-debianchangelog-perl <none>     (no description available)

-- no debconf information