At the European Lisp Symposium in 2015, we presented a paper entitled
"First-class Global Environments in Common Lisp".  There are several
possible use cases for such environments.  In this presentation, we
investigate the use of such environments at run time for so-called
"sandboxing", i.e., to allow only a pre-selected set of functionalities
to be visible to application code.  In particular, we demonstrate the
main idea that allows such environments to be used with no performance
loss in almost all cases.