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+======Atomic Scopes======
+=====What is an atomic scope?======
+An //atomic scope// is the period of time during which local variables live.
+At the beginning of an //atomic scope// there are no local variables, and
+at the end of that //atomic scope//, all local variables are destroyed.
+Thus, the //atomic scope// defines the availability and lifetime of a local
+variable.
+When the client starts up, it creates a global //atomic scope// that lives
+for the entire lifetime of the client.  Thus it is always possible to create
+local variables, even from the input prompt.
+From time to time (see the list below), the client creates new atomic scopes.
+When a new atomic scope is created, any local variables that were visible in
+the previous atomic scope are hidden and can no longer be accessed!  The
+client arranges for this by using a stack.  Each atomic scope can see only
+those local variables on its position in the stack.
+Eventually an atomic scope ends, and then its local variables are destroyed.
+They cannot be recovered once the atomic scope ends.  Because the scopes are
+stored as a stack, atomic scopes are destroyed in a strict Last-In-First-Out
+order.
+======The list of atomic scopes======
+  * Commands executed by [[defer]]
+  * Files loaded by the [[pf loader]]
+  * [[exec]] -line, -error, or -end callbacks
+  * Commands executed by [[wait]] -cmd
+  * User-created key[[bind]]ing callbacks
+  * Commands executed by [[queue]]
+  * Commands executed by [[timer]]
+  * User-created [[set]] callbacks
+  * [[Who]] -line or -end callbacks
+  * [[Ison]] -oncmd, -offcmd, or -end callbacks
+  * [[Userhost]] -cmd callbacks
+  * [[ON]] callback bodies
+  * Each time an [[alias]] is executed
+======Sharing local variables:======
+[future expansion]
+======History:======
+Local variables first appeared in EPIC4pre0.009