The legal values are
[list_begin definitions]
[def "[const none], [const -]"]
Draw no arrowheads, at neither end of the line.

[def "[const start], [const first], [const <-]"]
Draw an arrowhead at the beginning of the line, but not at its end.

[def "[const end], [const last], [const ->]"]
Draw an arrowhead at the end of the line, but not at its beginning.

[def "[const both], [const <->]"]
Draw arrowheads at both ends of the line.
[list_end]
