Added graph design ramblings.

doc/design.md

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+## Graph
+
+### 1. Support of math nodes (or non-AnimNodes in general)
+
+* Enables animators to add custom math for blend inputs or to adjust other inputs (e.g. LookAt or IK
+  targets).
+
+**Open Issues**
+
+1. When to do the evaluation? Two types of subgraphs:
+   a) Instant inputs (needed for blend node inputs) that have to be evaluated before
+   UpdateConnections
+   b) Processing nodes, e.g. for extracted bones.
+
+### 2. Support of multiple output sockets
+
+* E.g. extract Bone transform
+
+* Increases Node complexity:
+    * AnimOutput
+    * AnimOutput + Data
+    * Data
+
+(Data = bool, float, vec3, quat, ...)
+
+**Open Issues**
+
+1. Unclear when this is actually needed. Using more specific nodes that perform the desired logic
+   may be better (
+   c.f. https://dev.epicgames.com/documentation/en-us/unreal-engine/animation-blueprint-bone-driven-controller-in-unreal-engine).
+   Likely this is not crucial so should be avoided for now.
+
+### 3. Multi-skeleton evaluation
+
+Use case: riding on a horse, interaction between two characters.