The expression it writes into it.
The EXTEND this was read off, so that a caller can ask utils/certainlyBoundVars!cpMetaOf what
holds where the bind is evaluated rather than at the top of the chain. That difference is
load-bearing: ?y ∈ cVars of the whole input is also satisfied by a bind further up the chain writing
?y, which is precisely a ?y this bind reads unbound.
vars(e), cached: every licence reads it.
The variable the bind writes.
One
BIND(expression AS variable)lifted out of an EXTEND chain. The unit a pull-up decides about is the bind rather than the node carrying it, so nothing here refers back to the chain it came out of.