+/* The third argument to F() is a "prototype". For most prototypes, the first
+ letter (before the _) represents the return type and each other letter
+ (after the _) is an argument type. The types are:
+
+ - "m": A matrix of unrestricted dimensions.
+
+ - "d": A scalar.
+
+ - "v": A row or column vector.
+
+ - "e": Primarily for the first argument, this is a matrix with
+ unrestricted dimensions treated elementwise. Each element in the matrix
+ is passed to the implementation function separately.
+
+ The fourth argument is an optional constraints string. For this purpose the
+ first argument is named "a", the second "b", and so on. The following kinds
+ of constraints are supported. For matrix arguments, the constraints are
+ applied to each value in the matrix separately:
+
+ - "a(0,1)" or "a[0,1]": 0 < a < 1 or 0 <= a <= 1, respectively. Any
+ integer may substitute for 0 and 1. Half-open constraints (] and [) are
+ also supported.
+
+ - "ai": Restrict a to integer values.
+
+ - "a>0", "a<0", "a>=0", "a<=0".
+
+ - "a<b", "a>b", "a<=b", "a>=b".
+*/