Why isn't there a key equivalent to variable-binding-locus? As
in, a way to figure out where a particular keybinding is coming
from. For example,

    M-x key-binding-locus C-j

would evaluate to 'lisp-interaction-mode-map, which is the first
map in which C-j was found.


In terms of implementation, the process for finding a binding is
described in (info "(elisp) Searching Keymaps"):

    ...Here is a pseudo-Lisp description of the order and
    conditions for searching them:

     (or (cond
          (overriding-terminal-local-map
           (FIND-IN overriding-terminal-local-map))
          (overriding-local-map
           (FIND-IN overriding-local-map))
          ((or (FIND-IN (get-char-property (point) 'keymap))
       (FIND-IN-ANY emulation-mode-map-alists)
       (FIND-IN-ANY minor-mode-overriding-map-alist)
       (FIND-IN-ANY minor-mode-map-alist)
       (if (get-text-property (point) 'local-map)
           (FIND-IN (get-char-property (point) 'local-map))
         (FIND-IN (current-local-map))))))
         (FIND-IN (current-global-map)))

So implementing key-binding-locus would only be a small tweak of
the key lookup code: the first time you find the key, just return
the map you found it in, rather than the command it's supposed to
call.

Thanks,
Brian