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

fusl/src/math/atanh.c

+#include "libm.h"
+
+/* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
+double atanh(double x)
+{
+        union {double f; uint64_t i;} u = {.f = x};
+        unsigned e = u.i >> 52 & 0x7ff;
+        unsigned s = u.i >> 63;
+        double_t y;
+
+        /* |x| */
+        u.i &= (uint64_t)-1/2;
+        y = u.f;
+
+        if (e < 0x3ff - 1) {
+                if (e < 0x3ff - 32) {
+                        /* handle underflow */
+                        if (e == 0)
+                                FORCE_EVAL((float)y);
+                } else {
+                        /* |x| < 0.5, up to 1.7ulp error */
+                        y = 0.5*log1p(2*y + 2*y*y/(1-y));
+                }
+        } else {
+                /* avoid overflow */
+                y = 0.5*log1p(2*(y/(1-y)));
+        }
+        return s ? -y : y;
+}