Added recursion example. Fixes #346

examples/example024.scad

@@ -0,0 +1,32 @@
+// Menger Sponge
+// By Nathan Hellweg, Emmett Lalish and Marius Kintel May 13, 2013
+// CC-BY-SA license
+
+// Size of edge of sponge
+D=100;
+// Fractal depth (number of iterations)
+n=3;
+
+module menger() {
+  difference() {
+    cube(D, center=true);
+    for (v=[[0,0,0], [0,0,90], [0,90,0]])
+      rotate(v) menger_negative(side=D, maxside=D, level=n);
+  }
+}
+
+module menger_negative(side=1, maxside=1, level=1) {
+  l=side/3;
+  cube([maxside*1.1, l, l], center=true);
+  if (level > 1) {
+    for (i=[-1:1], j=[-1:1])
+      if (i || j)
+        translate([0, i*l, j*l])
+          menger_negative(side=l, maxside=maxside, level=level-1);
+  }
+}
+
+difference() {
+  rotate([45, atan(1/sqrt(2)), 0]) menger();
+  translate([0,0,-D]) cube(2*D, center=true);
+}