unrealistic 2D volleyball simulation

 Slime Volley is a 2D arcade-oriented volleyball simulation, in the spirit of
 some Java games of the same name.
 Two teams, 1-3 players each, try to be the first to get 10 points. This happens
 when the one ball touches the floor on the other side of the net.
 There can be 1 to 8 balls in game.
 Each player use 4 keys, 2 direction keys, one for jump and one for changing its
 skin.