# an input is something the player does
#   it can be instantaneous (keydown, keyup)
#   it can be have duration (keypressed)
#   it can be analog (joystick)
#   it can be positional (mouse, touch)
# a control is a function the game provides
#   it allows external control over the game
#   it 
# a binding is the configuration for which inputs map to which controls
#   it can be 1-to-1
#   it can be left unmapped
#   it may not apply in all contexts
# a context is a state in which a particular set of bindings are active
#   it is a tree
#   multiple contexts can be active, but only one will match and order matters
#   it can add bindings, remove bindings, or redefine bindings from the context above it
#   the root of the tree represents bindings that are universally active (unless removed)
#   different branches of the tree apply to different sections of the game
#   for example, an inventory screen may have its own context with additional or redefined bindings
#   several context branches may inherit bindings from a common parent