Ask for autocompletion at a particular spot in the code, returning possible things that can be completed at that location.
Ask for autocompletion at a particular spot in the code, returning
possible things that can be completed at that location. May try various
different completions depending on where the index
is placed, but
the outside caller probably doesn't care.
The returned index gives the position from which the possible replacements should be inserted.
Nice wrapper for the presentation compiler.