diff --git a/Test-problems.md b/Test-problems.md
index f77e6e9..702f7e6 100644
--- a/Test-problems.md
+++ b/Test-problems.md
@@ -57,7 +57,23 @@ Fix two or more “pin” points in 2-space or 3-space. Constrain a movable “p
 
 #### Notes
 
-To avoid topological issues, it would be simplest to model the length of the string as a whole-number linear combination of the distances from the points to the pencil. To get an ellipse, use two pins and fix the sum of the distances (with unit coefficients).
+To avoid topological issues, it would be simplest to model the length of the string as a whole-number linear combination of the distances from the points to the pencil. To get an ellipse, use two pins and fix the unweighted sum of the distances.
+
+### Generalized conic: triangle of directrices
+
+#### Source
+
+No source found yet, but it seems to be in the spirit of [generalized conics](https://en.wikipedia.org/wiki/Generalized_conic) as described on Wikipedia.
+
+#### Statement
+
+Fix three “pin” points in 3-space. Constrain a movable “pencil” point by fixing the surface area of the tetrahedron whose vertices are the pins and the pencil.
+
+Equivalently, fix three “directrix” lines in 3-space that intersect pairwise, forming a triangle. Constrain a movable “pencil” point by fixing the sum of the distances from the directrices to the pencil.
+
+#### Notes
+
+To get an ellipse or an ellipsoid, use two pins in 2- or 3-space and fix the area of the triangle whose vertices are the pins and the pencil.
 
 ## Hierarchical constraints