a planning system based on the event calculus

Automatically retractable chalk and plumb line assembly including a housing having two compartments, one of the compartments being formed with a closable first opening through which chalk is receivable in the one compartment, the other of the compartments being sealed against penetration by chalk from the one compartment, a reel disposed in the one compartment, a line wound on said reel and unwindable so as to extend out of the housing through a second opening formed in the one compartment and means disposed in the other compartment for rewinding the line on the reel.