Historically, the common law has frowned on the use of defeasible estates as it interferes with the owners' enjoyment of their property and as such has made it difficult to create a valid future interest.
Durational language such as "to A as long as the property is used for a park" creates a fee simple determinable and a possibility of reverter.
An attempt to create such an interest is construed as a fee simple subject to condition subsequent (see below), and a person who would have possibility of reverter at common law will instead have a right of entry.
A fee simple subject to an executory limitation is an estate that ends when a specific condition is met and then transfers to a third party.
Also, unlike a fee simple subject to a condition subsequent, B then automatically gains the interest in Blackacre and does not only have a mere right to sue for re-entry.
The future interest is called a "right of reentry" or "right of entry", and the property only reverts to the original grantor if he exercises this right.
To exercise right of entry, the holder must take substantial steps to recover possession and title, for example, by filing a lawsuit.