COMPOUND LITERAL
Node* new_top = malloc(sizeof(*new_top));
*new_top = (Node) { .next = NULL, .ch = ch };  // Compound literal
└──────┘   └───────────────────────────────┘
  Node                  Node

Do not use:
 - new_top -> next = NULL;
 - new_top -> ch   = ch;

Using compound literal helps avoid bugs.

D.R.Y. Rule - Don't Repeat Yourself

Example:
    // BEFORE
    if(*a_top == NULL) {  // stack is initially empty
        *a_top = new_top;
    }
    else {                // stack is not empty
        (*a_top) -> next = new_top;   // make the existing top refer to the new top
        *a_top = new_top;             // new top becomes the top of the stack
    }

    // AFTER
    if(*a_top != NULL) {  // stack is not initially empty
        (*a_top) -> next = new_top;   // make the existing top refer to the new top
    }
    *a_top = new_top; // new top becomes the top of the stack
}

CAUTION:  new_top should refer to the old_top, not the other way around.
    // WRONG
    (*a_top) -> next = new_top;   // make the existing top refer to the new top

    // RIGHT
    new_top -> next = *a_top;   // make the new top refer to the existing top

BOOLEAN VARIABLES / VALUES

bool not int
true not 1
false not 0
#include <stdbool.h>  at the top


RECURSIVE vs. ITERATIVE SOLUTION