+ * Casting any lifted type to {\tt Any}
+
+ * Casting {\tt Any} back to the real type
+
+ * Casting an unboxed type to another unboxed type of the same size
+
+ * Casting between two types that have the same runtime representation. One case is when
+ the two types differ only in "phantom" type parameters, for example
+ {\tt Ptr Int} to {\tt Ptr Float}, or {\tt [Int]} to {\tt [Float]} when the list is
+ known to be empty. Also, a {\tt newtype} of a type {\tt T} has the same representation
+ at runtime as {\tt T}.
+
+ Other uses of {\tt unsafeCoerce\#} are undefined. In particular, you should not use
+ {\tt unsafeCoerce\#} to cast a T to an algebraic data type D, unless T is also
+ an algebraic data type. For example, do not cast {\tt Int->Int} to {\tt Bool}, even if
+ you later cast that {\tt Bool} back to {\tt Int->Int} before applying it. The reasons
+ have to do with GHC's internal representation details (for the congnoscenti, data values
+ can be entered but function closures cannot). If you want a safe type to cast things
+ to, use {\tt Any}, which is not an algebraic data type.
+
+ }
+
+-- NB. It is tempting to think that casting a value to a type that it doesn't have is safe
+-- as long as you don't "do anything" with the value in its cast form, such as seq on it. This
+-- isn't the case: the compiler can insert seqs itself, and if these happen at the wrong type,
+-- Bad Things Might Happen. See bug #1616: in this case we cast a function of type (a,b) -> (a,b)
+-- to () -> () and back again. The strictness analyser saw that the function was strict, but
+-- the wrapper had type () -> (), and hence the wrapper de-constructed the (), the worker re-constructed
+-- a new (), with the result that the code ended up with "case () of (a,b) -> ...".