procedure main()
stack := []                                     # new empty stack
push(stack,1)                                   # add item
push(stack,"hello",table(),set(),[],5)          # add more items of mixed types in order left to right
y := top(stack)                                 # peek
x := pop(stack)                                 # remove item
write("The stack is ",if isempty(stack) then "empty" else "not empty")
end

procedure isempty(x)           #: test if a datum is empty, return the datum or fail (task requirement)
if *x = 0 then return x        #  in practice just write *x = 0 or *x ~= 0 for is/isn't empty
end

procedure top(x)               #: return top element w/o changing stack
return x[1]                    #  in practice, just use x[1]
end