Class Tree<T>

Derived classes must implement the following interface:

public:
  Real underlying(Size i, Size index) const;
  Size size(Size i) const;
  Size descendant(Size i, Size index, Size branch) const;
  Real probability(Size i, Size index, Size branch) const;

and provide a public enumeration

enum { branches = N };

where N is a suitable constant (2 for binomial, 3 for trinomial...)