void insert(HashTable ht, Item it)
{
   assert(ht->nitems < ht->nslots);
   int N = ht->nslots;
   Item *a = ht->items;
   Key k = key(it);
   int i, j, h = hash(k,N);
   for (j = 0; j < N; j++) {
      i = (h+j)%N;
      if (a[i] == NoItem) break;
      if (eq(k,key(a[i]))) break;
   }
   if (a[i] == NoItem) ht->nitems++;
   a[i] = it;
}