TODO: write docs

If o is not a polygon, throw an IllegalArgumentException with an appropriate message; otherwise, returns o. Macro, so exception is thrown from the calling function.

If o is not a sequence of polygons, throw an IllegalArgumentException with an appropriate message; otherwise, returns o. Macro, so exception is thrown from the calling function.

If o is not a triangle, throw an IllegalArgumentException with an appropriate message; otherwise, returns o. Macro, so exception is thrown from the calling function.

Return a polygon like triangle but with a key :gradient whose value is a unit vector representing the gradient across triangle.

Return a polygon constructed from these vertices.

True if o satisfies the conditions for a polygon. A polygon shall be a map which has a value for the key :vertices, where that value is a sequence of vertices.

Return a rectangle, with edges aligned east-west and north-south, whose south-west corner is the vertex vsw and whose north-east corner is the vertex vne.

Return a canonicalised facet (i.e. a triangular polygon) with an added key :centre whose value represents the centre of this facet in 3 dimensions. This only works for triangles, so is here not in walkmap.polygon. It is an error (although no exception is currently thrown) if the object past is not a triangular polygon.

True if o satisfies the conditions for a triangle. A triangle shall be a polygon with exactly three vertices.