CIGAL Reference Manual, Chapter 4 (Variables): stack
usage: stack = values ; Write to the STACK cautiously!
STACK is an internally defined long array variable providing direct access to essentially all of CIGAL's main memory data space: HSTACK. Because almost all of CIGAL's active working memory is stored in either STACK or HSTACK, these variables let's you look at (and modify) the program's internal data directly.
Obviously, changing values within STACK (or HSTACK(4)) can have dire consequences for CIGAL's function and should only be done with great caution.