The combination of of inheritance for the mixin itself, plus the use of a mixin, and added to the layered tests makes this rather difficult to follow. It looks like ```Test_register__CoordSig_scalar``` has a mixin (```_MergeTest_coordprops```) that defines a test that uses a helper defined in the mixin, which relies on a test defined in the superclass of the mixin which itself uses another test in the top level mixin. Phew.
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