Say that behavior variables start out not kept

Behaviors.md

@@ -285,8 +285,8 @@ playing use the same two words.
   back what it is given. With a start node in the graph, its On start nodes stay out of the call.
 - **Keep** (a switch on each variable of the called behavior, in its inspector): a kept variable
   picks up where the last call **from this node, on this object** left it; the others start at
-  their default every call. A variable added to a behavior starts out kept, and one from a project
-  saved before the switch existed starts out not kept, so older projects run as they did. Two call
+  their default every call. Every variable starts out not kept, a new one as well as one from a
+  project saved before the switch existed; Keep is switched on by hand. Two call
   nodes running the same behavior keep a state each, and so do two objects running the same graph;
   a call inside a call has one per outer node. It follows today's declarations: a variable added
   since starts at its default, a removed one is dropped, and one whose kind changed starts over.