Write about manipulating observables and setting regulators

Command-language.md

@@ -48,6 +48,31 @@ 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.
 
+#### Manipulating observables
+
+```
+Angle a b ~ 30
+```
+
+Push the angle between `a` and `b` toward 30°. This is analogous to the manipulations you might do by dragging elements in the display. It doesn’t create a regulator, because it’s referring to `Angle a b` as an observable.
+
+#### Setting regulators
+
+```
+Angle a b @ 30
+```
+
+Set the regulator for the angle between `a` and `b` to 30°, creating the regulator if it doesn’t already exist.
+
+```
+unset Angle a b
+```
+```
+Angle a b @ unset
+```
+
+Unset the regulator for the angle between `a` and `b`.
+
 ### Identifiers
 
 #### Assigning identifiers