/// <summary>
/// Expands given inference level via back chaining.
/// </summary>
/// <param name="parent">The parent to be expanded.</param>
/// <returns>The expansion.</returns>
private IEnumerable<InferenceLevel> expand(InferenceLevel parent)
{
foreach (var requirement in parent.GetRequirements())
{
if (!_wildcardToLevel.ContainsKey(requirement))
yield return getState(parent, requirement);
}
}
