Hedger Wang 222594cb44 [Navigator] A method subtract to compute the diff between stack mutation.
Summary:
When mutation of a stack happens, we'd like to compute the diff of the stacks (before and after) so that
we can know which routes are removed in the new stack.

This diff adds a new method `substract` which does what we need.
2015-08-04 16:16:17 -08:00
..
2015-07-24 13:16:25 -08:00