The two scratch registers are parked at the marker #
DescriptiveComplexity.Draw.Data.ixLegStB_fields says a leg of the spine
leaves the mirror, the working register, the bottom mark, the stage tracks and
the last-pass flag alone. It says nothing about the two scratch registers, SAV
and TARGET, because a leg does write them – and the evaluation's exit asks that
they be back at the marker when the spine is over (hsavL, htgtL).
This file closes that: an atom parks both at the marker when it is a stage atom
and touches neither otherwise (ixKindEndSt), so a state entered with them
parked leaves with them parked, and the property rides the matrix's chain, the
round, the VAL loop's thread and the whole branched spine. The marker being the
empty address, the entry state has them parked for free.