Introspecting yas tab binding

[Psionic K]

As far as I can tell, Yas minor mode uses a menu item to define the tab binding.

In this case I knew where to look, and the keymap was simple.  I could
guess yas's minor mode keymap was responsible by turning the minor
mode off.  Still, it was not a  regular binding, and so I almost
missed it.

I do think I should be able to figure this out without the prior
knowledge.  I debugged the call but saw no evidence on the stack.
`where-is' for the command did not find the binding.  Command log and
lossage are both dead ends.

As far as I can tell, yas is using this menu hack to implement a form
of conditional binding, intercepting the key sequence conditionally
with a `:filter' function in the menu binding.

How should I have discovered this?