The exponent to be applied to (inc (:z (unit-vector from to))) of a path segment to calculate the gradient-related part of the cost of traversal. Dynamic, because we will want to tune this.

The (expected to be negative) exponent to be applied to the number of traversals of a path to compute the road bonus. Paths more travelled by should have larger bonuses, but not dramatically so - so the increase in bonus needs to scale significantly less than linearly with the number of traversals. Dynamic, because we will want to tune this.

Search superstructure s and return a sequence of barriers, if any, which obstruct traversal from vertex from to vertex to.

Find the best traversable path which links the vertices from and to in this superstructure s, or nil if there are none.

TODO: should return the cost of crossing this barrier, initially mainly a watercourse, on the axis from vertex from to vertex to. in the context of superstructure s. If there’s a bridge, ferry or other crossing mechanism in s at the intersection of the vertex and the barrier, then the penalty should be substantially less than it would otherwise be.

Return a sequence like frontier with all of these candidates which are not already members either of frontier or of rejects appended.

Compute the per-unit-distance cost of traversing this edge.

TODO: write docs

Calculate the road bonus of the edge represented by the vertices from, to, in the context of the superstructure s. Obviously there only is such a bonus if there actually is an existing thoroughfare to use. Road bonuses scale with some fractional exponent of the number of traversals which have been made of the road segment in question.

TODO: write docs

True if this object can be considered as part of the walkmap.

Return the traversal cost of the edge represented by the vertices from, to, in the context of the superstructure s. It is legitimate to pass nil as the to argument, in which case the result will be zero, in order to allow reduce to be used to compute total path costs.

TODO: write docs