def function(self,p):
raise NotImplementedError<EOL>
Function to draw a pattern that will then be scaled and rotated.

Instead of implementing __call__ directly, PatternGenerator
subclasses will typically implement this helper function used
by __call__, because that way they can let __call__ handle the
scaling and rotation for them.  Alternatively, __call__ itself
can be reimplemented entirely by a subclass (e.g. if it does
not need to do any scaling or rotation), in which case this
function will be ignored.