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

fusl/src/math/atanhl.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double atanhl(long double x)
+{
+        return atanh(x);
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+/* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
+long double atanhl(long double x)
+{
+        union ldshape u = {x};
+        unsigned e = u.i.se & 0x7fff;
+        unsigned s = u.i.se >> 15;
+
+        /* |x| */
+        u.i.se = e;
+        x = u.f;
+
+        if (e < 0x3ff - 1) {
+                if (e < 0x3ff - LDBL_MANT_DIG/2) {
+                        /* handle underflow */
+                        if (e == 0)
+                                FORCE_EVAL((float)x);
+                } else {
+                        /* |x| < 0.5, up to 1.7ulp error */
+                        x = 0.5*log1pl(2*x + 2*x*x/(1-x));
+                }
+        } else {
+                /* avoid overflow */
+                x = 0.5*log1pl(2*(x/(1-x)));
+        }
+        return s ? -x : x;
+}
+#endif