Change your MSDscript implementation to use environments instead of substitution. The change includes

• adding an Env class;
• adding PTR(Env) as an argument to Expr::interp;
• removing the Expr::subst method; and
• adapting existing Expr::interp tests.

For tests, it’s almost certainly a good idea to define empty instead of writing new EmptyEnv() many times. You may even want to declare empty as a static variable of Env in "env.h":

CLASS(Env) {
  ....
  static PTR(Env) empty;
};

and initialize it "env.cpp" with

PTR(Env) Env::empty = NEW(EmptyEnv)();

Then, you can use Empty::env anywhere that "env.h" is included.