Why not?

src/cmd/map/libmap/cubrt.c

@@ -0,0 +1,30 @@
+#include <u.h>
+#include <libc.h>
+#include "map.h"
+
+double
+cubrt(double a)
+{
+	double x,y,x1;
+	if(a==0) 
+		return(0.);
+	y = 1;
+	if(a<0) {
+		y = -y;
+		a = -a;
+	}
+	while(a<1) {
+		a *= 8;
+		y /= 2;
+	}
+	while(a>1) {
+		a /= 8;
+		y *= 2;
+	}
+	x = 1;
+	do {
+		x1 = x;
+		x = (2*x1+a/(x1*x1))/3;
+	} while(fabs(x-x1)>10.e-15);
+	return(x*y);
+}