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A hook is a means to call a Scheme procedure when a specific event occurs. For example,
mouse-down-hook is a procedure that gets called when any mouse button is pressed. Hooks by default are set to a null function and do nothing. A user or developer can
set! a hook to some Scheme procedure. For example, a user can assign to
mouse-down-hook a procedure that picks an entity, highlights it, and initiates rubber band drag mode. The user can also assign to
mouse-up-hook a procedure that un-highlights the entity and terminates rubber band drag mode.
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