Quote: use toggle switches to display Sketchpad's abstractions; e.g., constraint is a circle and code letter with four arms to the constrained variables

Quotation Skeleton

[Besides picture elements,] certain very useful abstractions … are represented in the ring structure storage [e.g., the circle constraint]. … To make it possible for a user to … Not only does displaying abstractions make it possible … to aim at them with the light pen … be turned on or off selectively by toggle … constraints are displayed as shown in Figure 7. … The four arms of the constraint extend … to the variables constrained   Google-1   Google-2

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