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

fusl/src/math/atanhf.c

Index: fusl/src/math/atanhf.c
diff --git a/fusl/src/math/atanhf.c b/fusl/src/math/atanhf.c
new file mode 100644
index 0000000000000000000000000000000000000000..65f07c0f4f26e302f98ee4cc75b471c4204ee829
--- /dev/null
+++ b/fusl/src/math/atanhf.c
@@ -0,0 +1,28 @@
+#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;
+}