The words STRUCTURE and ENDSTRUCTURE are always used
in pairs. In the text between them, the top-of-stack
hold a value that is both the sizeof-struct and the
current-offset. ENDSTRUCTURE will save the last value in
the "structname" parameterpart CREATEd by STRUCTURE,
and SIZEOF will fetch the value from the "structname"
(so, SIZEOF is state-smart).