I've just checked in a test file for .WALK as spec'd in S12.
While I'm happy :canonical, and :super are correct (the first is just C3
by default and the second just those from the superclass) and I think
:breadth is also, I'm a little less sure on :preorder, :ascendant and
:descendant - mostly because :ascendent and :preorder work out the same
here, but I can't think of a case right now where they'd differ.
Comments and patches/commits to the test file welcome (plus an
implementation that currently passes this test file will be in Rakudo's
src/setting/Object.pm very shortly...)