ConstDrops the mask mark from names the caller binds to a value: while filter(d, k) is processed k has no
read yet. A name bound to a function keeps its mark, as filter(d, id > 2) next to id <- function(...)
still means the column. Hands back the masking call and the names it just unmarked, undefined when
name does not mask at all, so linkMasksToData need not ask a second time.
Links every name the data masks of calls supply to the data it comes from: id in filter(df, id > 2)
is a column of df, so it reads df. Run once every mark has settled, as adding the read makes
suppliedByMask false for the name; a call whose first argument is masked too has no data to link
against.
Whether the vertex is a name a data mask may supply, which is every name appearing in the mask.
Whether id is a name a data mask supplied, which the mark the masking call carries is what records.
Unlike suppliedByMask this survives linkMasksToData, so ask this one after the dataflow
is complete and that one while a call is still being processed.
Whether id is a name the data mask supplies, i.e. a use the caller does not bind itself.
Drops the non-standard-evaluation mark from the outgoing edges of id that which accepts, and reports them.
The ids an Unquote escape hands back to standard evaluation, the markers themselves excluded.
The parts of a call R does not evaluate the standard way.