struct nlist {             /* table entry: */
    struct nlist *next;    /* next entry in chain */
    char *name;            /* defined name */
    char *defn;            /* replacement text */
};

struct nlist *install(char *name, char *defn) {
    struct nlist *np;
    unsigned hashval;
    if ((np = lookup(name)) == NULL) { /* not found */
        np = (struct nlist *) malloc(sizeof(*np));
        if (np == NULL || (np->name = strdup(name)) == NULL)
            return NULL;
        hashval = hash(name);
        np->next = hashtab[hashval];
        hashtab[hashval] = np;
    } else /* already there */
        free((void *) np->defn); /*free previous defn */
    if ((np->defn = strdup(defn)) == NULL)
        return NULL;
    return np;
}

np->next = hashtab[hashval];
hashtab[hashval] = np;

hashtab[hashval]   -->  original_list

hashtab[hashval]   -->  original_list
np->next           -->  original_list

hashtab[hashval]  -->  *np
        np->next  -->  original_list