Create example of reading a zero-initialized array (#49)

src/example-archive/simple-examples/working/array_read.c

@@ -0,0 +1,44 @@
+/** Get the first element of a zeroed, nonempty array */
+int head(int *arr, unsigned long len)
+/*@
+requires
+    take arr_in = each(u64 i; i < len) {
+        Owned(array_shift<int>(arr, i))
+    };
+    each(u64 i; i < len) {
+        arr_in[i] == 0i32
+    };
+    len > 0u64;
+
+ensures
+    take arr_out = each(u64 i; i < len) {
+        Owned(array_shift<int>(arr, i))
+    };
+    each(u64 i; i < len) {
+        arr_out[i] == 0i32
+    };
+    return == 0i32;
+@*/
+{
+    unsigned long idx = 0;
+
+    // First, we apply `extract` to direct CN to the relevant element of the
+    // iterated resource `arr_in`, which it needs in order to verify the
+    // following read:
+    /*@ extract Owned<int>, idx; @*/
+
+    int hd = arr[idx];
+
+    // Now, we need to demonstrate that `hd` is zero in order to return it and
+    // satisfy the third `ensures` clause. To do so, we should think of our
+    // second `requires` clause as introducing a quantified constraint, which is
+    // approximately `forall i. len > i ==> arr_in[i] == 0` (`==>` being logical
+    // implication). In order to leverage this constraint, we need to
+    // instantiate `i` with our choice of index `idx`:
+    /*@ instantiate idx; @*/
+
+    // Now, our constraint has evolved into `len > 0 ==> arr_in[0] == 0`. From
+    // here, verification of this constraint can proceed automatically. (Recall
+    // that we already required that `len > 0u64`.)
+    return hd;
+}