tt(PUSHSUBST) can be used to (temporarily) suppress the interpretation of
tt(SUBST) definitions (the tt(SUBST) built-in command is covered
below, refer to its description for an example). 

tt(PUSHSUBST) expects one argument: an integral number which is either 0 or
non-zero (commonly: 1). After calling tt(PUSHSUBST)tt((0)) tt(SUBST)
definitions are not interpreted anymore; use tt(POPSUBST)tt(()) to revert to
the previous type of interpretation. Alternatively, tt(PUSHSUBST)tt((0)) can
be used to stack another level of tt(SUBST) interpretations on top of the
last-used one.

On a 64-bit computer the tt(PUSHSUBST) stack can hold slightly more than 60
tt(SUBST) interpretation levels. When more levels are pushed, the oldest levels
are silently forgotten. Calling tt(POPSUBST) once the tt(PUSHSUBST) stack is
empty results in activating the tt(SUBST) interpretations (and so a
stack-underflow error will not be encountered).
