//Environment newEnv = old.extend(new ValueBinding(binding.getName(), defValue));
}
@Override
public void evalDecl(Environment evalEnv, Environment declEnv) {
VarValueBinding vb = (VarValueBinding) declEnv.lookup(binding.getName());
if (definition == null) {
vb.assign(null);
return;
}
Value defValue = definition.evaluate(evalEnv);
vb.assign(defValue);
}