func filterSubpool(ghc githubClient, sp *subpool) *subpool {
	var toKeep []PullRequest
	for _, pr := range sp.prs {
		if !filterPR(ghc, sp, &pr) {
			toKeep = append(toKeep, pr)
		}
	}
	if len(toKeep) == 0 {
		return nil
	}
	sp.prs = toKeep
	return sp
}
func filterSubpool(ghc githubClient, sp *subpool) *subpool {
	var toKeep []PullRequest
	for _, pr := range sp.prs {
		if !filterPR(ghc, sp, &pr) {
			toKeep = append(toKeep, pr)
		}
	}
	if len(toKeep) == 0 {
		return nil
	}
	sp.prs = toKeep
	return sp
}
// filterSubpool filters PRs from an initially identified subpool, returning the
// filtered subpool.
// If the subpool becomes empty 'nil' is returned to indicate that the subpool
// should be deleted.