modified:   abstraction.hpp
	modified:   application.hpp
	modified:   term.hpp
	modified:   variable.hpp

	Added 'operator std::string() const' to abstract syntax nodes.
	modified:   de-bruijn.hpp
	modified:   term.hpp

	Added 'operator std::string() const' to de_bruijn clases.
	modified:   application.hpp
	modified:   de-bruijn.hpp
	new file:   syntax-example.cpp
	modified:   term.hpp

	Cast to std::string now prettier. :)
	Cosmetics.
	Done with environment classes. Code needs clean up.

	modified:   environment-example.cpp
	modified:   environment.hpp