const int max_len = 40;

struct ch_stack {
//data representation
   char  s[max_len]; 
   int   top;
   enum { EMPTY = -1, FULL = max_len - 1 };

//operations represented as member functions
   void  reset() { top = EMPTY; }
   void  push(char c) {
      assert(top != FULL);
      top++;
      s[top] = c;
   }
   char pop() {
      assert(top!= EMPTY);
      return s[top--];
   }
   char top_of() {
      assert(top!= EMPTY);
      return s[top];
   }
   bool empty()
     { return (top == EMPTY); }
   bool full()
     { return (top == FULL); }
};