This assert is wrong, because there may be a legimate need to query
a position that is currently hidden.

However, oddly enough, we haven't found a legitimate document yet that
triggers this, and it has pointed us to several interesting functions
that needed adapting to merged frames... so keep it a little while and
remove it (and return something other than COMPLETE_STRING from mapping
functions) later...

Change-Id: Ide2abf0c2ce054b75c917063a5d5aa8d13430426