This function appends the directory dir to the set userdictpath option if not already present. If prepend is 1, prepends dir to the user dictionary path, removing any later occurrence of the directory.

If there is an Arbortext-path\custom\dictionaries subdirectory at startup, the custom\dictionaries path is automatically prepended to the user dictionary path. Putting your user dictionary files in the custom\dictionaries subdirectory makes them automatically available, avoiding manual steps to add them to the path.