> this has a huge practical issue of knowing what A was, which could be a while back So why not put the burden of establishing a basis (A) on the person seekin

> this has a huge practical issue of knowing what A was, which could be a while back So why not put the burden of establishing a basis (A) on the person seeking to claim its benefit — who, in this case, is also best-positioned to gather such evidence? Should they keep poor records, they lose the benefit of that basis, and will instead pay taxes on (B-0).