If the user clicks on a stone in the outermost position (F), it should be able to slide around the invisible 13th circle (i.e. to the F positions on adjacent circles and around until they're blocked). As seen in this screenshot, the user has selected the stone at F4 to move, but the 13th-circle options are not available:

This is in game # 5797941e3a0ac6306cd86c11 .