In the MMR code, we have a notion of "position" and "location". A node's position is its index within the MMR. Its location is its index within the list of MMR leaves.

This MMR has nodes at positions 0, 1, 2, 3.

The mapping of location to position is:
0 --> 0
1 --> 1
2 --> 3

(Note that position 2 does not correspond to a location -- it isn't a leaf.)

    2
  /   \
 0     1     3

We use the position/location to index into MMRs, etc.

We use u64 to represent both locations and positions. This can be error prone -- it's easy to mistake a location for a position and vice versa.

We could make this less error prone by introducing Position and Location types. These could alias (or wrap) u64. This would allow functions to be parameterized on one type or the other, reducing the likelihood that they are misused. We can trivially implement From on these types to allow for ergonomic conversions between the types. This would replace helper functions leaf_num_to_pos and leaf_pos_to_num.