the entire variable name in braces. For instance, in `ab$cd-x',
`$cd' is treated as an environment variable.
-If WHEN-UNDEFINED is nil, references to undefined environment variables
-are replaced by the empty string; if it is a function, the function is called
-with the variable name as argument and should return the text with which
-to replace it or nil to leave it unchanged.
+If WHEN-UNDEFINED is omitted or nil, references to undefined environment
+variables are replaced by the empty string; if it is a function, the
+function is called with the variable's name as argument, and should return
+the text with which to replace it, or nil to leave it unchanged.
If it is non-nil and not a function, references to undefined variables are
left unchanged.