> Now (try-completion "" '(("") (""))) returns t.
 > Previously it returned "".

I attached a patch for the offending occurrence of `try-completion'.
Please test it.  There are two more occurrences that should be checked
though.  And there's an additional problem: When, in *scratch*, I type

(defma )

do M-x PC-lisp-complete-symbol and choose say `defmath', the _entire_
contents of the buffer preceding defma are erased.  I think this is due
to the fact that complete.el doesn't set `completion-base-size'
appropriately.  Should this be corrected too?