Restore a minimal definition of execution frame, since other parts of

the manual refer to it.

XXX Not sure that it belongs in this section, or that the concept is
particularly important for writing documentation.  Perhaps references
to the frame should be removed entirely.

Doc/ref/ref4.tex

@@ -25,6 +25,12 @@ is a code block.  The string argument passed to the built-in function
 The expression read and evaluated by the built-in function
 \function{input()} is a code block.
 
+A code block is executed in an \dfn{execution
+frame}\indexii{execution}{frame}.  A frame contains some
+administrative information (used for debugging) and determines where
+and how execution continues after the code block's execution has
+completed.
+
 A \dfn{scope}\index{scope} defines the visibility of a name within a
 block.  If a local variable is defined in a block, it's scope includes
 that block.  If the definition occurs in a function block, the scope