* function: cstack_t->pop to remove the top char from the stack
* function: cstack_t->top to test if the top char is a given char
* function: cstack_t->empty to test if the stack is empty
* function: cstack_t->pop to remove the top char from the stack
* function: cstack_t->top to test if the top char is a given char
* function: cstack_t->empty to test if the stack is empty