Did you get an answer yet?

I'm suspicious that what you want is
  preceding-sibling::c[1]


> Assume a document structure like the one below. The "c" and "d"
> elements are at the same level inside of "b" which is inside of
> "a". Currently, I can find the first "c" in the "b" that encloses
> "d". But that is too far away. I want to find the closest preceding
> "c" within the surrounding "b" when processing the template for
> element "d".
> 
> When executing the template for "d", my current path attempt is
> (ancestor::a[1]//c)[1]

> But, if there is no preceding "c" in the "b" element, I'd like to
> detect that, and not find a "c" in any other "b". If I use just
> "preceding::c", I might find a "c" in some other "b", which I don't
> want.
> <a>
>    ...
>    other elements
>    ...
> </a>
> <a>
>    ...
>    other elements
>    ...
> </a>
> <a>
>    <b>
>    ...
>    </b>
>    ...
>    <b>
>       ...
>       <c/>   <---- My path gets this "c"
>       ...
>       <c/>
>       ...
>       <c/>   <---- I want this "c" when processing "d"
>       ...
>       <d/>
>       ...
>       <c/>
>       ...
>    </b>
>    ...
>    <b>
>    ...
>    </b>
> </a>
> <a>
>    ...
>    other elements
>    ...
> </a>
> ...