A random number engine adaptor
(commonly shortened to adaptor)
a of type A
is a random number engine
that takes values
produced by some other random number engine,
and applies an algorithm to those values
in order to deliver a sequence of values
with different randomness properties.
An engine b of type B adapted in this way
is termed a base engine
in this context.
The expression a.base() shall be valid and shall return a
const reference to a's base engine.