message: Map out first example
diff: @@ -5,4 +5,31 @@ Introductory examples
  Here is a collection of introductory examples showing Primal primitives and
  operations.
  +- instantiate a point, a ray, several triangles, a bounding box, a plane
  +- get triangles' area and normal; test for degeneracy
  +- make a visualization (Geomview?)
  +With these primitives, we can perform some geometric operations.
  +
  +- clip triangle against bbox to get polygon
  +- find which side of the plane a triangle point lies on
  +- find intersection point btw ray and triangle
  +- find the closest point from another triangle corner to another tri, and
  + the squared distance btw points
  +
  +A spatial index can be used when a code compares each primitive in a collection
  +to every other primitive, such as when checking if a triangle mesh intersects
  +itself. The following naive implementation is straightforward but runs
  +in $O(n^2)$ time, where $n$ is the number of triangles.
  +
  +- naive implementation: compare each triangle to every other tri
  +
  +A code should be able to skip the call to ``intersect`` for widely-separated
  +primitives. The UniformGrid is designed for this optimization.
  +
  +- UniformGrid example. For each triangle,
  + - find its bounding box
  + - get the bins intersecting the bounding box
  + - iterate over the contents of each bin, testing for intersection
  +
  +Find examples for the MortonIndex and the BVH tree.
  