Use the following functions to directly access GCL stacks.
(SI:VS i) Returns the i-th entity in VS.
(SI:IHS-VS i) Returns the VS index of the i-th entity in IHS.
(SI:IHS-FUN i) Returns the function of the i-th entity in IHS.
(SI:FRS-VS i) Returns the VS index of the i-th entity in FRS.
(SI:FRS-BDS i) Returns the BDS index of the i-th entity in FRS.
(SI:FRS-IHS i) Returns the IHS index of the i-th entity in FRS.
(SI:BDS-VAR i) Returns the symbol of the i-th entity in BDS.
(SI:BDS-VAL i) Returns the value of the i-th entity in BDS.
(SI:SUPER-GO i tag)
Jumps to the specified tag established by the TAGBODY frame at
FRS[i]. Both arguments are evaluated. If FRS[i] happens to be
a non-TAGBODY frame, then (THROW (SI:IHS-TAG i) (VALUES)) is
performed.