Add the ring of polyhedra problem

Test-problems.md

@@ -103,7 +103,7 @@ You can also set up and solve the problem by hand. A good way to do this is to f
 
 #### Source
 
-A version of the ["Frugal firepower"](#frugal-firepower-optimizing) problem that doesn't require hierarchical constraints.
+A version of the [“Frugal firepower”](#frugal-firepower-optimizing) problem that doesn't require hierarchical constraints.
 
 #### Statement
 
@@ -111,7 +111,27 @@ Place five unit spheres tangent to each other so that their centers form either
 
 #### 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.
+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.
+
+### Ring of polyhedra
+
+#### Source
+
+Rhombic ring of icosahedra
+
+- **Author:** Robert
+- **Published:** [“A Rhombic Ring of Icosahedra, Leading to a Rhombic Dodecahedron Made of Icosahedra.”](https://robertlovespi.net/2015/09/15/a-rhombic-ring-of-icosahedra-leading-to-a-rhombic-dodecahedron-made-of-icosahedra/) RobertLovesPi (September 15, 2015).
+
+#### Statement
+
+Choose a rigid ring of polyhedra. Assemble it by constraining distances between vertices or angles between faces.
+
+#### Notes
+
+Here are some examples of interesting rings:
+
+- A ring of eight dodecahedra
+- A [rhombic ring](https://robertlovespi.net/2015/09/15/a-rhombic-ring-of-icosahedra-leading-to-a-rhombic-dodecahedron-made-of-icosahedra/) of eight icosahedra
 
 ## Exploring configuration spaces