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

fusl/src/math/atanhf.c

+#include "libm.h"
+
+/* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
+float atanhf(float x)
+{
+        union {float f; uint32_t i;} u = {.f = x};
+        unsigned s = u.i >> 31;
+        float_t y;
+
+        /* |x| */
+        u.i &= 0x7fffffff;
+        y = u.f;
+
+        if (u.i < 0x3f800000 - (1<<23)) {
+                if (u.i < 0x3f800000 - (32<<23)) {
+                        /* handle underflow */
+                        if (u.i < (1<<23))
+                                FORCE_EVAL((float)(y*y));
+                } else {
+                        /* |x| < 0.5, up to 1.7ulp error */
+                        y = 0.5f*log1pf(2*y + 2*y*y/(1-y));
+                }
+        } else {
+                /* avoid overflow */
+                y = 0.5f*log1pf(2*(y/(1-y)));
+        }
+        return s ? -y : y;
+}