Completeness of context-sensitive rewriting

Restrictions of rewriting may turn normal forms of some terms unreachable, leading to incomplete computations. Context-sensitive rewriting (csr) is the restriction of rewriting that only permits reductions on arguments selected by a replacement map μ, which associates a subset μ(f) of argument indices with each function symbol f. Hendrix and Meseguer defined an algebraic semantics for Term Rewriting Systems (TRSs) executing csr that can be used to reason about programs written in programming languages like CafeOBJ and Maude, where such replacement restrictions can be specified in programs. Semantic completeness of csr was also defined. In this paper we show that canonical replacement maps, which play a prominent role in simulating rewriting computations with csr, are necessary for completeness in important classes of TRSs.