Merges this snip with the given snip, returning null if the snips cannot be merged or a new merged snip otherwise. This method will only be invoked if both snips are from the same class and both have the wx:const-snip-can-append flag.
If the returned snip does not have the expected count, its count is forcibly modified. If the returned snip is already owned by a another administrator, a surrogate snip is created.
The snip's buffer is usually internally locked for reading when this method is called (see also Locks).