subclasses

        # Class.subclasses

(from ruby core)
---
    subclasses -> array

---

Returns an array of classes where the receiver is the direct superclass
of the class, excluding singleton classes. The order of the returned
array is not defined.

    class A; end
    class B < A; end
    class C < B; end
    class D < A; end

    A.subclasses        #=> [D, B]
    B.subclasses        #=> [C]
    C.subclasses        #=> []