for absDir, keepDir := range keepDirs {
		nKept := 0
		for _, dir := range haveDirs[absDir] {
			if dir == keepDir { // (note that dir is always non-empty)
				nKept++
			} else {
				wf.DropUse(dir)
			}
		}
		if keepDir != "" && nKept != 1 {
			// If we kept more than one copy, delete them all.
			// We'll recreate a unique copy with AddUse.
			if nKept > 1 {
				wf.DropUse(keepDir)
			}