"persistent" here means state that outlives any particular evaluation,
i.e. everything that exist outside of a call to eval() and friends. do
note that this leave EvalState nearly empty, containing only things of
importance to evaluation *itself*, e.g. the call depth tracker. do not
worry about the self-reference ctx member. nothing to see here, go on.
Change-Id: Ie8d0af3f09d33902f81e0c36a1096dd9f334a537
eldritch horrors
8e74d1ceeb