It had been since law school that I had thought about fee simples. But, in law school, a great deal of time was wasted on it. The great legal authority of modern times, Wikipedia, had this to say: "If previous grantors of a fee simple estate do not create any conditions for subsequent grantees to own the conveyed property in fee simple title, which is commonly the case these days, then the title is called fee simple absolute. Other fee simple estates in real property include fee simple defeasible (or fee simple determinable) estates. A defeasible estate is created when a grantor places a condition on a fee simple estate. Upon the happening of a specified event, the estate may become void or subject to annulment. Two types of defeasible estates are the fee simple determinable and the fee simple subject to a condition subsequent. If the grantor uses durational language in the condition such as "to A as long as the land is used for a park" then upon the happening of the specified event, the estate will automatically terminate and revert to the grantor or the grantor's estate; this is called a fee simple determinable. If the grantor uses language such as "but if alcohol is served" then the grantor or the heirs have a right of entry, but the estate does not automatically revert to the grantor; this is a fee simple subject to a condition subsequent. In the United States many of these concepts have been modified by statute in some states."
That last sentence encapsulates the difference between the practice of law and the study of law in most American law schools. We spent hours parsing different types of estate interests. I suppose the point was made but I recall those sessions as being agonizing. As between listening to my property professor quizzing us about future interests or eating glass, I'd just ask that the glass be ground fine and served with a martini. In Washington, the rules are laid out in the Revised Code of Washington, most notably Chapters 64 and 65. In my practice, I have never seen a "fee simple determinable." We spent hours on the rules against perpetuities, yet the Revised Code of Washington abolished the rule. One little tidbit my trusts and estates professor omitteed. Same with the Rule in Shelley's case.
Alas, if only I knew then what I know now.