$addinstancefunction = $fromform->modulename."_add_instance";
$updateinstancefunction = $fromform->modulename."_update_instance";
if (!empty($fromform->update)) {
// ...
$returnfromfunc = $updateinstancefunction($fromform);
Functions in mod/XXX/lib.php are often called like this, when you don't know in advance which modules you will be calling into.
However, you are right, in general, a good cross-reference tool is invaluable.