NULL_CATEGORY = 1           # 1 is a special number here.
                            # Do not change unless you really know what you are doing.
FIRST_PRIME_NUMBER = 2      # 2 is a special number here.
                            # Do not change unless you really know what you are doing.
FOUND_IN_CHILDREN = "FOUND IN CHILDREN"

HIERARCHY = "hierarchy"
LAYERS = "layers"
OPTIONS = "options"