In C++, it's possible to include source in the header file by means of
inline functions. Sometimes, it's very tempting to do so; for example, it
had been easier to implement the functions empty and full as
inline functions. It goes without saying that this reveals no small part of
the implementation and is in general bad practice.
Not all C++ compilers support boolean types, but g++ does. If you use some
other compiler and are making a hush application, including the file
hush/bool.h will provide the same boolean types as g++.