tt(POPSUBST) is used to revert to a previous level of interpretation of
tt(SUBST) definitions. Refer to the descriptions of the tt(PUSHSUBST) and
tt(SUBST) builtin commands below for details. 

There is no limit to the number of times tt(POPSUBST) can be called. Once the
`tt(PUSHSUBST) stack' is empty tt(SUBST) definitions are automatically 
interpreted  (so no stack-underflow error is ever encountered).
