state_mask/2 state_mask(?State, ?Mask)
Translates between events state fields that consist of
button and modifier specifications and the corresponding
integer bitmasks.
State is unified with a term of the form
state(Buttons, Modifiers), where Buttons is the
buttons specification used by buttons_mask/2 and Modifiers is
the modifiers specification used by modifiers_mask/2.
Mask is the integer bitmask representing the given set of buttons and modifiers.