Using the functionality of existing graph styles

This example demonstrates how linestyles, symboltypes, colors, and other
decorations can be changed in existing graph styles. ... We make use of the
`changelist' feature as an argument when using a style. Generally, all items in
the attribute lists can be iterable `changelists'. If the end of such a list is
reached, the changelist restarts from its beginning (see e.g. the colors which
are used several times).

The `attr.stroked.clear' in the symbol deletes the default property of the
symbols, which says to stroke the symbol outline.