Add the combinatorial version of "Frugal firepower"

Test-problems.md

@@ -99,6 +99,20 @@ As of pull request #84, the function [`engine::examples::realize_irisawa_hexlet`
 
 You can also set up and solve the problem by hand. A good way to do this is to first impose all the tangencies and then start fixing diameters. Right now, the engine isn't very robust, so you'll have to adjust the spheres as you set up the problem to help the engine satisfy each additional constraint.
 
+### Frugal firepower (combinatorial)
+
+#### Source
+
+A version of the [Frugal Firepower](#frugal-firepower-optimizing) problem that doesn't require hierarchical constraints.
+
+#### Statement
+
+Place five unit spheres tangent to each other so that their centers form either a square pyramid or a triangular bipyramid. Build a rectangular box whose faces are parallel to the coordinate axes. By exploring different patterns of tangency between the spheres and the walls of the box, try to make the box enclose the spheres while minimizing the sum of its length, width, and height.
+
+#### Notes
+
+One might reasonably hope that solving this problem will provide a solution of the original Frugal Firepower problem. The idea is that a box that solves the original problem should have "maximal contact" with the spheres, and should therefore be determined by its tangencies with the spheres.
+
 ## Exploring configuration spaces
 
 ### The configuration space of equiangular, equilateral hexagons