Right now, successorsetverb only needs successors to compute the verb. But we
will want use additional information (effect-flags and operation) in the near
future to compute a better verb.
Add the markers parameter now so extensions (like Evolve) could wrap the
function and start experimenting around better obsfate verbs.
As successorsetverb now takes both successorset and markers parameters, rename
it to obsfateverb, successorsetandmarkersverb was too long.