Update the documentation with the new syntax for lsystems.
Return the symbol env alone rather than the package senv.
Raise an exception when a negative integer is given as generation number.
Separate lsystem evaluation on smaller operation in the engine.
Move all stream stuff into a separate file and expose them through the interface. Update the documentation accordingly.
Small fix in the documentation, works better with '-colorize-code'.
Expose the internal engine as a functor through the interface.