Add Irisawa's hexlet

Test-problems.md

@@ -23,7 +23,40 @@ for some positive $r$, $s$. Show that $A$, $B$, $C$, $D$, $E$, $F$ must be the v
 
 #### Coverage
 
-At pull request #84, you can place six points in a rough planar hexagon, impose the distance constraints for some chosen $r$ and $s$, and see that the assembly becomes an equiangular planar hexagon, which seems to be rigid when you nudge the points.
+As of pull request #84, you can place six points in a rough planar hexagon, impose the distance constraints for some chosen $r$ and $s$, and see that the assembly becomes an equiangular planar hexagon, which seems to be rigid when you nudge the points.
+
+### Irisawa's hexlet
+
+#### Source
+
+- **Author:** Irisawa Shintarō Hiroatsu
+- **Published:** Donated to Samukawa Shrine in Sagami Province as a *sangaku* in 1822. Recorded by Irisawa’s teacher Uchida Itsumi in the book *Kokon sankan* (*Mathematics, past and present*) in 1832.
+- **Access:** Abe Haruki. [“Japan’s ‘Wasan’ Mathematical Tradition.”](https://www.nippon.com/en/japan-topics/c12801/) Nippon.com (April 30, 2024).
+
+#### Statement
+
+Abe gives the following translation of the problem as stated in *Kokon sankan*. The length unit [*sun*](https://en.wikipedia.org/wiki/Japanese_units_of_measurement#Length) is unrelated to the “sun” sphere.
+
+> As shown in the figure, place two spheres (“sun” and “moon”) inside a larger external sphere, such that both touch the external sphere’s inner surface, and then create a chain of connected spheres in the remaining space within the external sphere. If the diameters of the external, sun, and moon spheres are 30, 10, and 6 *sun* respectively, and the diameter of the sphere marked *kō* is 5 *sun*, find the diameters of the other spheres.
+
+Here's an alternative statement.
+
+Six spheres form a cyclic chain, each one externally tangent the next. Two spheres, the “sun” and the “moon,” are each externally tangent to all the chain spheres. An outer sphere encloses and is tangent to all the other spheres. Fix the diameters of these spheres:
+
+Sphere | Diameter
+---|---
+Outer | 30
+Sun | 10
+Moon | 6
+One chain sphere | 5
+
+What are the diameters of the other chain spheres?
+
+#### Coverage
+
+As of pull request #84, the function [`engine::examples::realize_irisawa_hexlet`](../src/commit/2adf4669f47ab8f2bff8d64ac011a2bd09f632fa/app-proto/src/engine.rs#L526-L573) sets up and solves this problem for use in the unit test [`irisawa_hexlet_test`](../src/commit/2adf4669f47ab8f2bff8d64ac011a2bd09f632fa/app-proto/src/engine.rs#L715-L727) and the example [`irisawa-hexlet.rs`](https://code.studioinfinity.org/StudioInfinity/dyna3/src/commit/2adf4669f47ab8f2bff8d64ac011a2bd09f632fa/app-proto/examples/irisawa-hexlet.rs).
+
+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.
 
 ## Exploring configuration spaces