Start writing down command language ideas

Command-language.md

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+The dyna3 command language will provide a text-based way to interact with assemblies, parallel to the graphical interface. A mostly-commutative fragment of it will also be used to serialize assemblies. Here are some ideas about how it might be designed.
+
+## Design goals
+
+- Quick and comfortable to use during interactive sessions.
+- Minimal punctuation.
+
+## Syntax ideas
+
+### General principles
+
+- **Commands have verbs.** Each command should contain an implicit or explicit verb. It should be easy to learn to look at a command and see what the verb is.
+- **Creation by presumption.** A command that presumes the existence of an element or regulator should create that element or regulator, as long as it’s specified unambiguously and it doesn’t exist already.
+
+### Elements
+
+#### Creating elements
+
+```
+Sphere
+```
+
+Create a sphere with default parameters and a default ID. The verb in this command is implicit: “[let there be] a sphere.” We can think of this as [copula omission](https://en.wikipedia.org/wiki/Zero_copula). Creating a sphere through the graphical interface should have the same result.
+
+### Observables and regulators
+
+#### Referring to regulators
+
+Given two elements `a` and `b`, we can refer to the angle between them as `Angle a b`. If the angle is being regulated, we can refer to the regulator in the same way.
+
+We could make regulator references more visually coherent, and help distinguish them from identifiers, by requiring parentheses: `Angle(a b)`. This would be especially helpful for observables with short names, like an `X` observable for the $x$ coordinate of a point. In an expression like `X a`, the observable and the element are visually similar. The form `X(a)` makes it clear that `X` is a regulator.
+
+#### Creating regulators
+
+```
+Angle a b
+```
+
+```
+regulate Angle a b
+```
+
+Create a regulator for the angle between elements `a` and `b`. The implicit copula option is shorter, and more analogous to how you create an element, but it might introduce confusion about whether we’re referring to an observable or a regulator. The option with the explicit verb `regulate` makes it more clear that we’re referring to a regulator.
+
+```
+Angle a Sphere
+```
+
+By the **creation by presumption** principle, this should create a regulator for the angle between the element `a` and a new sphere.
+
+### Identifiers
+
+#### Assigning identifiers
+
+```
+a is Sphere
+```
+```
+Sphere named a
+```
+
+Create a sphere, as described above, and set its identifier to `a`.
+
+```
+r is Angle a b
+```
+```
+Angle a b named r
+```
+
+Assign the identifier `r` to the regulator for the angle between `a` and `b`, creating the regulator if it doesn’t exist already.
+
+```
+a is c
+```
+```
+c named a
+```
+
+Assign the a new identifier, `a`, to the object with the identifier `c`.
+
+#### Nesting identifier assignments
+
+```
+r is Angle a b is Sphere
+```
+```
+Angle a Sphere named b named r
+```
+
+It seems like this should assign the identifier `r` to the angle between `a` and a new sphere with identifier `b`. Readability is very poor for both versions!