In older versions of FCS, when deriving a state, all the allocated stacks
were first allocated to a temporary memory, then modified and then
looked up or registered in the stack collection.
Starting from version 2.6.x, a different, faster scheme was enacted:
Every stack is marked with its own flag. Once modified, the stack is
temporarily allocated and its flag set.
The non-marked stacks are kept as they are, while the marked stacks
are looked up in the stack collection.
This is a variant of the Copy-on-Write (COW) technique.