(make-ra-stack) ;returns a newly created empty stack. (empty-ra-stack? ra-stack) ;returns #T if ra-stack is empty, ;otherwise #F. (height ra-stack) ;returns the height (i.e., number of ;elements) in ra-stack. (top-minus ra-stack offset) ;returns the element which is offset items ;below the top of ra-stack, provided ;0 ≤ offset < (height ra-stack). ;In particular, (top-minus ra-stack 0) ;returns the top of ra-stack, provided ;ra-stack is non-empty. (pop! ra-stack) ;removes the top element of ra-stack, ;provided ra-stack is non-empty. ;Return value is the modified ra-stack. (push! ra-stack item) ;pushes item onto the top of ra-stack. ;Return value is the modified ra-stack.