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+## Separation
+
+Elements don't interact directly with each other, and constraints don't interact directly with each other either. In other words, the graph of direct interactions among elements and constraints is bipartite.
+
+### Contrasts
+
+Elements | Observables
+---|---
+Naturally seen as geometric objects | Naturally seen as quantities
+No notion of valence; can interact with any number of constraints | Many important observables have fixed valence
+An assembly consisting entirely of elements seems meaningful and potentially useful | An assembly consisting entirely of observables seems hard to interpret
+
+***To do:** Expand and clarify list of contrasts.*
+
+## Unification
+
+Can we realize constraints as geometric objects? For example:
+
+* A real angle constraint might be seen as a pair of infinitesimal ribbons crossing at a fixed angle along a circle
+* An imaginary angle constraint might be seen as a circle that passes perpendicularly through a pair of spheres, with a fixed cross ratio.
+
+Maybe being subject to a constraint can then be seen as a kind of incidence.
+
+***To do:** Explore ideas for unifying elements and observables in spite of the contrasts described above.*
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