Delete code doesn't seem to work correctly

[mccarthy-bsm]

It appears that the sequence insert 0,1,2,3,4,5,6,7 followed by delete 0,1,2,3,4,5 fails when removing
key 5, when flip_colors is called for a node without a right child. Also, prior to the failure, the tree
becomes highly imbalanced to the left.

It's possible I messed something up compiling (it doesn't compile cleanly on gcc/msys), but it appears
that there's a case where an excess rotate_left occurs on delete.