anchor.diff:  ----
   Compute the local active information with a history length `k`.
   
  -If `ai == NULL`, then an array is allocated and returned.
  -
   *Examples:*
   
   One initial condition:
  
positive.code: ----
  inform_error err = INFORM_SUCCESS;
  int const series[9] = {0,0,1,1,1,1,0,0,0};
  double *ai = inform_active_info(series, 1, 9, 2, 2, NULL, &err);
  assert(inform_succeeded(&err));
  // ai ~ {-0.193, 0.807, 0.222, 0.222, -0.363, 1.222, 0.222}
  ----
  
  Two initial conditions:
  [source,c]
  ----
  inform_error err = INFORM_SUCCESS;
  int const series[18] = {0,0,1,1,1,1,0,0,0, 1,0,0,1,0,0,1,0,0};
  double ai[14];
  inform_local_active_info(series, 2, 9, 2, 2, ai, &err);
  assert(inform_succeeded(&err));
  // ai ~ { 0.807, -0.363, 0.637, 0.637, -0.778, 0.807, -1.193,
  //        0.807,  0.807, 0.222, 0.807,  0.807, 0.222,  0.807 }
  ----
  ****
  
  
negative.code: ----
  inform_error err = INFORM_SUCCESS;
  int const series[9] = {0,0,1,1,1,1,0,0,0};
  double ai = inform_active_info(series, 1, 9, 2, 2, &err);
  assert(inform_succeeded(&err));
  // ai ~ 0.305958
  ----
  
  Two initial conditions:
  [source,c]
  ----
  inform_error err = INFORM_SUCCESS;
  int const series[18] = {0,0,1,1,1,1,0,0,0,
                          1,0,0,1,0,0,1,0,0};
  double ai = inform_active_info(series, 2, 9, 2, 2, &err);
  assert(inform_succeeded(&err));
  // ai ~ 0.359879
  ----
  ****
  