// cstack.pri

private:
    int  top;             // Subscript of current top item (or -1 if empty)
    char data[SMAX_LENG];  // the items in the stack

    // CLASSINV: -1 <= top && top < SMAX_LENG
    // ABSTRACTION MAP: if top is -1, the sequence is empty (<>),
    // otherwise the sequence is <data[top], data[top-1], ... ,data[0]>.