have some other changes on the mainline after W.
If you merge the updated side branch (with D at its tip), none of the
-changes made in A nor B will be in the result, because they were reverted
+changes made in A or B will be in the result, because they were reverted
by W. That is what Alan saw.
Linus explains the situation:
$ git revert W
This history would (ignoring possible conflicts between what W and W..Y
-changed) be equivalent to not having W nor Y at all in the history:
+changed) be equivalent to not having W or Y at all in the history:
---o---o---o---M---x---x-------x----
/