[plt-scheme] Code for catching exceptions in embedding applications

Great thanks and now, I hope, the last question in the series: I
cannot find `exn?' and `exn-message' in the basic environment, i.e.
scheme_lookup_global(scheme_intern_symbol("exn?"), environment)
returns NULL.

I tried to `require' scheme instead of scheme/base but this did not help.

Can you advise what is required for code to work.

    environment = scheme_basic_env();
    declare_modules(environment);
    scheme_namespace_require(scheme_intern_symbol("scheme/base"));
    exn_p = scheme_lookup_global(scheme_intern_symbol("exn?"), environment);