tool to display library dependencies?

[Drew Adams]

This has probably been asked before, but I couldn't find it.

Is there a tool somewhere that will look at one or more Emacs Lisp libraries
and output a tree or list of its library dependencies? An Emacs command that
does this would be great.

For example: Library A requires B, which requires C and D. I'd like to be
able to determine all of the libraries that A ultimately requires - in this
case B, C, and D. A dependency tree would be even better:

 A-B-+-C
     |
     +-D

I realize that it would be problematic to take autoloads into account, but
just dealing with the explicit `require's would still be useful. And it
would be good if such a tool/command distinguished somehow between hard and
soft (`(require nil t)') `require's.

 - Drew