A system for tracking players that enter and leave a stage, a-la D3.
Implementation detail: This code is hampered by the lack of Elm's type classes. Note the following:
{ entering : List a
, continuing : List a
, leaving : List a
}
A data-structure that maintains the difference list of entering, continuing and leaving players.
empty : Delta a
A default starting state for Deltas.
generation : List a -> Delta a -> Delta a
Update the delta by inspecting a list of players.