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class ExpectedFailures(object):
    def __init__(self):
        self._failures = set()

    def _can_trust_results(self, results):
        if not results or not results.failure_limit_count():
            return False
        return len(results.failing_tests()) != 0 and len(results.failing_tests()) != results.failure_limit_count()

    def failures_were_expected(self, results):
        if not self._can_trust_results(results):
            return False
        return set(results.failing_tests()) <= self._failures

    def shrink_expected_failures(self, results, run_success):
        if run_success:
            self._failures = set()
        elif self._can_trust_results(results):
            # Remove all expected failures which are not in the new failing results.
            self._failures.intersection_update(set(results.failing_tests()))

    def grow_expected_failures(self, results):
        if not self._can_trust_results(results):
            return
        self._failures.update(results.failing_tests())
        # FIXME: Should we assert() here that expected_failures never crosses a certain size?