How to debug elisp code written using the "deferred" package

[Jeff Rush]

I'm new to elisp programming but I've decided to tackle work on the
org-toodledo
package, which uses the deferred.el package.  I'm getting an error in
one of the
deferred handlers but can't figure out which one.  I need to be able to
have the
error me into the Emacs debugger upon an error signal.

1. How can I set the 'deferred:debug' variable to t from within the
org-toodledo
    package?  Simply doing the following does not work.

    (setq deferred:debut t)

2. How can I set the deferred package to drop into the debugger upon an
error?
    The following does not seem to work:

   (setq deferred:debug-on-signal t)

I also tried editing the source for deferred.el directly to change those
values but
still cannot get into the debugger.  I just get a brief error message in
the message
without about insufficient args for a format string with no clue as to
where it
happens.

Any advice please?  I've tried to find more advance documentation on the
deferred
package, without luck.

Thanks!

-Jeff