Vector::Add() (and similar methods) can be implemented via Vector::Add(T&& x=T()). That reduces amount of code.

Similar methods include:

Array::Add()
BiVector::AddHead(), BiVector::AddTail()
AMap::Add(K&& k), Add(const K& k), FindAdd(const K& k), FindAdd(K&& k), Put(const K& k), e.t.c.
ArrayMap::Add(const K& k), Add(K&& k)
FixedAMap::Add(const K& k)

It looks like half of methods of AMap can be eliminated ...

Just my two cents ...