Fix beamChain example and add negative polyRoundExtrude example

examples/negative-polyRoundExtrude.scad

@@ -0,0 +1,24 @@
+// negative polyRoundExtrude example
+
+include <Round-Anything-1.0.4/polyround.scad>
+
+extrudeRadius = 0.8;
+extrudeHeight = 2;
+tiny = 0.005; // tiny value is used to stop artifacts from planes lining up perfectly
+
+radiiPoints=[
+    [-7, -3,  0 ],
+    [7,  -3,  0 ],
+    [0,  6,   1 ] // top of the triagle is rounded
+];
+negativeRadiiPoints=[
+    [-3, -1,  0 ],
+    [3,  -1,  0 ],
+    [0,  3,   1 ] // top of the triagle is rounded
+];
+
+difference() {
+    polyRoundExtrude(radiiPoints,extrudeHeight, extrudeRadius, extrudeRadius,fn=20);
+    translate([0,0,-tiny])
+        polyRoundExtrude(negativeRadiiPoints,extrudeHeight+2*tiny, -extrudeRadius, -extrudeRadius,fn=20);
+}