[Boost-users] [ptr_container] Why is the key passed to ptr_map by non-const reference?

Before anyone refers me to the FAQ at http://www.boost.org/libs/ptr_container/doc/faq.html#why-does-ptr-map-t-inse... - I read it. While it's clear why a reference is used I don't understand why it is a *non-const* reference. Any reason why it's not const? Boris