How many names this binds; an overlay counts what it adds on top of its base.
StaticemptyThe empty frame, the same one every time, so an environment defining nothing allocates nothing.
StaticofA frame over map, frozen, so a write forks rather than reaching back into it.
An overlay is walked far more often than written, so the flattened view is kept until a write drops it; it is filled from overlaidEntries, whose contents and order it therefore has.
OptionalthisArg: unknownA frame the caller may write to, holding what this one holds. A small frame is copied outright; a big one becomes the base of an overlay, see OverlayThreshold. An overlay is big by construction, so it overlays again until the stack reaches DefaultMaxOverlayDepth, where a copy flattens it.
Kept once walked, like entries; a prefix scan asks every layer for its names.
Returns an iterable of values in the map
One frame's bindings. Taken from a map it is frozen, since the map is somebody else's.
A frame either owns all of its bindings, or holds what was written on top of a
baseit shares with whoever it forked from, see Frame.forWrite. The two read the same from the outside.