Backtracking in generalized control settings

Backtracking is a powerful conceptual and practical technique in programming. However, its application in general has been limited to global control over recursive programs. In this paper we explore through several examples the coherence and utility of applying backtracking in more general control settings, notably coroutine environments. The examples include: (i) a dual tree walk program using coroutine-managed backtracking subsystems, (ii) a context-free language intersection tester using bi-level hierarchical backtracking, and (iii) a minimizing computer job scheduler using backtracking in a simulation language setting. Full programs are given for each example, expressed in a PASCAL extension offering both coroutines and nondeterministic control.