// using its result.
func (state *debugState) mergePredecessors(b *Block, blockLocs []*BlockDebug, previousBlock *Block, forLocationLists bool) (abt.T, bool) {
	// Filter out back branches.
	var predsBuf [10]*Block

	preds := predsBuf[:0]
	locs := blockLocs[b.ID]

	blockChanged := !locs.everProcessed // the first time it always changes.
	updating := locs.everProcessed

        def step = new HasPrefixAndSuffixPatternStep("pre", "suf", CASE_SENSITIVE)

        expect:
        step.matches("pre-suf")
        step.matches("presufsuf")
        step.matches("presuf")
        !step.matches("")
        !step.matches("pre")
        !step.matches("pre-su")
        !step.matches("pre-s")
        !step.matches("suf")
        !step.matches("re-suf")
        !step.matches("e-suf")