+ *
+ * Use: Checks the primality of @p@. If @p@ is prime, then this
+ * function returns nonzero; if @p@ is really composite then it
+ * %%\emph{probably}%% returns zero, but might not.
+ *
+ * Currently, this function uses the Baillie--PSW test, which
+ * combines a single Miller--Rabin test with witness 2 with a
+ * single Frobenius test with parameters chosen using
+ * Selfridge's `Method A'. No composites are known which pass
+ * this test, though it's conjectured that infinitely many
+ * exist.