Widening a global tagged union into a wider tagged slot copied
nothing of the box; treat the global ident as a tagged source and
copy the full box from g(SB) through the nested arm. Both-wrong pair:
cstage spills frame garbage as the box (filed task #44, residual
non-deterministic so the rows assert divergence only). The non-nested
SUBSET-widen shape remains open as task #49 (site comment at the
fall-through). Review item #51.
Hojun-Cho
e416885d96