return new CachingSpec(spec);
                }
            });
        } catch (ExecutionException e) {
            throw UncheckedException.throwAsUncheckedException(e.getCause());
        }
    }

    private class CachingSpec implements Spec<FileTreeElement> {
        private final Spec<FileTreeElement> spec;

        expect:
        spec instanceof AndSpec
        spec.specs.size() == 2
        spec.specs[0] instanceof CachingPatternSpecFactory.CachingSpec
        spec.specs[1] instanceof NotSpec
        spec.specs[1].sourceSpec instanceof CachingPatternSpecFactory.CachingSpec
    }