source: CIVL/examples/verifyThis/pairInsertSort_sorted_loop-invariants-simple.cvl

#include <stdio.h>
#pragma CIVL ACSL
$input int N = 6; // upper bound on n
$input int n; // size of array
$assume(1<=n && n<=N);
$input int LEFT;
$input int RIGHT;
$assume(1<=LEFT && LEFT < RIGHT && RIGHT<n-1);

int main() {
  int a[n];

  // assumption: a[LEFT-1] is less than every thing of right of LEFT:
  $assume($forall (int j : LEFT .. RIGHT) a[j] >= a[LEFT-1]);
  int left = LEFT, right = RIGHT;
  int k = left;
  
  left++;
  /*@ loop invariant left <= right + 2;                                  // initially left <= right, left <= right+2 at termination
    @ loop invariant LEFT <= k && k <= RIGHT+1 && k == left - 1;         // initially LEFT <= k < RIGHT, LEFT <= k <= RIGHT + 1 at termination
    @ loop invariant \forall int i; LEFT <= i && i <= RIGHT ==>
    @                               a[i] >= a[LEFT-1];
    @ loop invariant \forall int j; LEFT <= j && j < k ==>
    @                               a[j-1] <= a[j];
    @*/
  for (;left <= right;) {
    int a1 = a[k], a2 = a[left];
    if (a1 < a2) {
      a2 = a1; a1 = a[left];
    }
    k--;
    while (a1 < a[k]) {
      a[k + 2] = a[k];
      k--;
    }
    a[++k + 1] = a1;
    k--;
    while (a2 < a[k]) {
      a[k + 1] = a[k]; 
      k--;
    }
    a[k + 1] = a2;
    k = ++left;
    left++;
  }
  // [LEFT, RIGHT-1] is guaranteed to be sorted, but if element at [RIGHT] is 
  // sorted depends on if length of the sub-array is even.
  $assert( $forall (int i : LEFT .. RIGHT-2) a[i] <= a[i+1] );
}