def center_middle(r, window_size):
res = copy.copy(r)<EOL>mid = res.start + (len(res) / <NUM_LIT:2>)<EOL>res.start = mid - (window_size / <NUM_LIT:2>)<EOL>res.end = res.start + window_size<EOL>return res<EOL>
Center a region on its middle and expand it to window_size bases.

:return: the new region.