ThingLab[ Borning, A., ``The Programming Language Aspects of ThingLab, a Constraint-Oriented Simulation Laboratory'', ACM Trans. on Programming Languages and Systems, vol. 3, no. 4 (Oct. 1981), pp. 353-387.] is an interactive graphical simulation system based on Smalltalk.

• Objects have ports that can be connected graphically.

• If an object is modified interactively, it propagates changes to its ports, and thus to objects to which it is connected.

• Features of objects can be ``anchored'' to prevent them from changing.

• An object may do search by attempting to change different port values to make itself consistent.

• Problems are solved by value propagation or by relaxation.

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