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

fusl/src/math/acosh.c

Index: fusl/src/math/acosh.c
diff --git a/fusl/src/math/acosh.c b/fusl/src/math/acosh.c
new file mode 100644
index 0000000000000000000000000000000000000000..badbf9081e644ff9df80bd2ae86cfc32e32a888b
--- /dev/null
+++ b/fusl/src/math/acosh.c
@@ -0,0 +1,24 @@
+#include "libm.h"
+
+#if FLT_EVAL_METHOD==2
+#undef sqrt
+#define sqrt sqrtl
+#endif
+
+/* acosh(x) = log(x + sqrt(x*x-1)) */
+double acosh(double x)
+{
+	union {double f; uint64_t i;} u = {.f = x};
+	unsigned e = u.i >> 52 & 0x7ff;
+
+	/* x < 1 domain error is handled in the called functions */
+
+	if (e < 0x3ff + 1)
+		/* |x| < 2, up to 2ulp error in [1,1.125] */
+		return log1p(x-1 + sqrt((x-1)*(x-1)+2*(x-1)));
+	if (e < 0x3ff + 26)
+		/* |x| < 0x1p26 */
+		return log(2*x - 1/(x+sqrt(x*x-1)));
+	/* |x| >= 0x1p26 or nan */
+	return log(x) + 0.693147180559945309417232121458176568;
+}