add to xBar, also refactor

helper/halfspace.scad

@@ -5,10 +5,7 @@ include <./common.scad>
 
 module halfspace(vpos, p) {
     translate(p)
-    align(a=[0,0,1], b=vpos)
+    align(a=[0,0,-1], b=vpos)
     translate(v=[0,0,-inf/2])
         cube(size=[inf, inf, inf], center=true);
-}
-
-
-halfspace(vpos=[1,1,1], p=[10,10,10]);
+}

helper/math.scad

@@ -13,9 +13,20 @@ function mirror4XY(midpoint, offsetX, offsetY)  =
 
 
 module align(a,b) {
+
+    echo("a", a);
+    echo("b", b);
     rot_axis = cross(a,b);
-    angle = acos(a*b/ (norm(a)*norm(b)));
+
+    if (rot_axis == 0) {
+        error("Can't align - provided vectors are parallel");
+    }
+
+    echo("rot_axis", rot_axis);
+
+    angle = acos(a*b/(norm(a)*norm(b)));
+    echo("angle", angle)
 
     rotate(v=rot_axis, a=angle)
         children(0);
-}
+}