if neither source nor target file exists then both get created empty.
if the target already exists and the source does not then synchronization should happen from the target back to the source.
if the target does not exist and the source does then synchronization happens from the source to the target.
if both the source and target file exist then there is a choice:
    If the file is internal (the default): source -> target (the presumption is that the file will be managed in the .dotfiles)
    If the file is encrypted: then target -> source
    If the file is external: target -> source (the presumption is that the file is normally changes by some external force)