+ * Normaly, you use the "default" implementations, obtained
+ * through relevant function calls. These functions return
+ * implementations that are deemed "best" for the current
+ * platform, where "best" means "fastest within constant-time
+ * implementations". Selecting the default implementation is a
+ * mixture of compile-time and runtime checks.
+ *
+ * Nevertheless, specific implementations may be selected
+ * explicitly, e.g. to use code which is slower but with a
+ * smaller footprint.
+ *