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A program that allowed users to draw on a screen with a lightpen. It supported constraints (e.g. drawing a constrainedellipse produced a circle). It also had some computer aideddesign features (e.g. computing loads on beams).Sketchpad was the subject of Ivan E. Sutherland's 1963 MITPhD thesis, which opened the field of computer graphics. Itwas the progenitor of computer drawing packages like MacDrawor Adobe Illustrator. There is a film of Sketchpad inaction.It solved constraints using value inference and introducedthe "ring" list structure.["Sketchpad: A Man-Machine Graphical Communication System",I.e. Sutherland, MIT Lincoln Lab, TR 296 (Jan 1963)].[Sammet 1969, p. 678].