'stack' on|off|free
Several functions which return values to the user might now place them also on a software-stack (used by area,dist,ds,enq,gtol,length,prnt,valu,volu and many more). The “valu” command is able to read this values from the stack and place them in variables. The 'free' parameter will empty the stack but does not open or close it.

See also “if” and “while”.