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

fusl/src/math/atanhl.c

Index: fusl/src/math/atanhl.c
diff --git a/fusl/src/math/atanhl.c b/fusl/src/math/atanhl.c
new file mode 100644
index 0000000000000000000000000000000000000000..87cd1cdb5f325f01cf12d19594efbcead93472b1
--- /dev/null
+++ b/fusl/src/math/atanhl.c
@@ -0,0 +1,35 @@
+#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