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

fusl/src/complex/catanhl.c

Index: fusl/src/complex/catanhl.c
diff --git a/fusl/src/complex/catanhl.c b/fusl/src/complex/catanhl.c
new file mode 100644
index 0000000000000000000000000000000000000000..a5dd538e44535ccc4961eadb593df3f1461a649b
--- /dev/null
+++ b/fusl/src/complex/catanhl.c
@@ -0,0 +1,14 @@
+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double complex catanhl(long double complex z)
+{
+	return catanh(z);
+}
+#else
+long double complex catanhl(long double complex z)
+{
+	z = catanl(CMPLXL(-cimagl(z), creall(z)));
+	return CMPLXL(cimagl(z), -creall(z));
+}
+#endif