Get the nth placeholder link, if it exists, or a default value if it does not.
The idx parameter indicates the nth placeholder link, starting from 0.
The str parameter is the text to replace the placeholder with, or “[unused]” if there is
no link at that index.
The elem parameter is an optional location that can be used to link to an element. If it
exists, elem.asSome() will return the location. Otherwise, elem.isNone() will hold.
predicate getExtra(int idx, string str, OptionalLocation elem)