One end of a string maybe be tied to the other end of the same string. Yet most standard plan representations identify operations as a mating of two different parts. With flexible parts it is common to mate parts in stages. A string may be attached to a reel early in the plan, but only wound up on the reel much later. But most planners allow the goal attachment between two parts to be either made or unmade, but not to have any intermediate states.
Operations are hierarchical in nature.
To tie a knot may be considered a primitive operation, but it also consists of a number operations of loop forming and string insertion, and pulling operations. This project is in its early stages. We will focus initially on string-like flexible parts, such as wires, hoses, strings, ropes, and cables. This is probably the single most important class of flexible parts, but our intent is to explore methods extendable to more general classes. We have proposed a representation method for the state of an assembly that is based on relations between features of parts.