+ """Simple bullet implementation.
+
+ Attributes:
+ x, y - current X/Y position
+ px, py - X/Y position prior to the last step
+ mx, my - X/Y axis-oriented speed modifier ("acceleration")
+ direction - direction of movement, in radians
+ speed - speed of movement, in units per frame
+ target - object with .x and .y fields for "aim" directions
+ vanished - set to true by a <vanish> action
+ rank - game difficulty, 0 to 1, default 0.5
+
+ Contructor Arguments:
+ x, y, direction, speed, target, rank - same as the attributes
+ actions - internal action list
+ parent - parent of actions, None for manually-created bullets
+
+
+ """