Add a "fork" of musl as //fusl.

fusl/src/math/asinh.c

+#include "libm.h"
+
+/* asinh(x) = sign(x)*log(|x|+sqrt(x*x+1)) ~= x - x^3/6 + o(x^5) */
+double asinh(double x)
+{
+        union {double f; uint64_t i;} u = {.f = x};
+        unsigned e = u.i >> 52 & 0x7ff;
+        unsigned s = u.i >> 63;
+
+        /* |x| */
+        u.i &= (uint64_t)-1/2;
+        x = u.f;
+
+        if (e >= 0x3ff + 26) {
+                /* |x| >= 0x1p26 or inf or nan */
+                x = log(x) + 0.693147180559945309417232121458176568;
+        } else if (e >= 0x3ff + 1) {
+                /* |x| >= 2 */
+                x = log(2*x + 1/(sqrt(x*x+1)+x));
+        } else if (e >= 0x3ff - 26) {
+                /* |x| >= 0x1p-26, up to 1.6ulp error in [0.125,0.5] */
+                x = log1p(x + x*x/(sqrt(x*x+1)+1));
+        } else {
+                /* |x| < 0x1p-26, raise inexact if x != 0 */
+                FORCE_EVAL(x + 0x1p120f);
+        }
+        return s ? -x : x;
+}