Member atria::xform::enumerate: Should be constexpr in C++14
Member atria::xform::zip: This should a constexpr in C++14
File signals.hpp: We could eventually flatten signals when the sucessors knows the transducer of its predecessor, this could be done heuristically.
Class spy_fn< MockT >: Support comparing the actual arguments. Keep generic interface using boost::any
Class structure< T >: doc
Class variant_spy: Add support for checking the actual values that were passed.