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

fusl/src/math/atanh.c

Index: fusl/src/math/atanh.c
diff --git a/fusl/src/math/atanh.c b/fusl/src/math/atanh.c
new file mode 100644
index 0000000000000000000000000000000000000000..63a035d7063ae2d5df2c31728e10e5d471209c79
--- /dev/null
+++ b/fusl/src/math/atanh.c
@@ -0,0 +1,29 @@
+#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;
+}