_Userspace_ libraries implement the common functionality
between DrScheme and DrScheme Jr.

The libraries _beginner.ss_, _intermediate.ss_, and _advanced.ss_
implement the additional function definitions that appear in the
initial namespace in the respective language levels. The other
language levels do not have any additional function definitions beyond
what is in the namespace automatically.

- beginner.ss adds:
  - mzlib's core libraries
  - simple-draw.ss
  - posn struture, without mutators

- intermediate.ss adds exactly what beginner.ss does

- advanced.ss adds:
  - mzlib's core libraries
  - the turtles, from turtle.ss in the graphics library, but only if
    the namespace already contains mred@
  - posn struture, including mutators

Each of the beginner.ss, intermediate.ss, and advanced.ss and
libraries returns a procedure that installs the corresponding bindings
into the current namespace.

The file _advancedr.ss_ contains a unit that exports all of the defintions
in advanced.ss.

The _simple-draw.ss_ library implements a simple drawing
library on top of sixlib. It uses _error.ss_.
