// epsi^a = empty ? public override RegEx <T> Derivative(T c) { return(RegExFactory.Empty <T>()); }
// [a...) = sum( {a}, {a+1}, {a+2}, ... ) // [a...)^x = sum( {a}, ... )^x = sum( {a}^x, {a+1}^x, ... ) // = epsilon if x >= a else empty public override RegEx <T> Derivative(T c) { return(Character.CompareTo(c) <= 0 ? RegExFactory.Epsilon <T>() : RegExFactory.Empty <T>()); }