ReadonlyargsReadonlydefinitionReadonlyregistryThe registry the edge was declared over (screenEdges<Registry>()): what
the target's handle type reads the screens that can open it from
(ScreenOpenersOf, ScreenItemOpenersOf). unknown for an
edge written without one: its target's origin is the generic
ScreenHandle.
ReadonlysourceThe source type of the edge's step: what the target's handle
type is built over — a tab definition entered by an opening source is
pushed (its own handle, with back), any other entry of it is a tab
switch (the tab's root screen), and a source type that
may be either is the conservative union of the two
(TabEntryReading).
What an edge remembers of its target, carried by the function its member returns: the target's definition (
definition) and the caller's arguments (args). It is never a value: the handle's bound actions read it to type the target's handle, itsoriginandbackread fromregistry.