What are first-class subroutines? What languages support them? What is a subroutine closure? What is it used for? How is it implemented? What is an object closure? How is it related to a subroutine...

What are first-class subroutines? What languages support them? What is a subroutine closure? What is it used for? How is it implemented? What is an object closure? How is it related to a subroutine closure? Describe how the delegates of C# extend and unify both subroutine and object closures.