zypper-aptitude from zypp:SLE-12-SP3-Branch project
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Add repository and install manually
For openSUSE Leap 42.3 Update run the following as root:
zypper addrepo https://download.opensuse.org/repositories/zypp:SLE-12-SP3-Branch/openSUSE_Leap_42.3_Update/zypp:SLE-12-SP3-Branch.repo zypper refresh zypper install zypper-aptitude