int fox_pop;
int geese_pop;
int next_fox_pop;
int next_geese_pop;
int n; //Year number

...
//0. Prime the loop
n = 0;
//Get fox_pop and geese_pop from the user for year 0

//Calculate the poplulations year by year until you get to total years.
while (n <= total_years)
{
    //Invariant: fox_pop and geese_pop are correct populations for year n
    // 1. Print the fox_pop and geese_pop if year is divisible by PRINT_FREQ

    //Assert: At this point, invariant is still true
    
    // 2. Calulate next years populations
    //    (next_fox_pop and next_geese_pop)

    //Assert: Invariant is still true
    //Assert: next_fox_pop and next_geese_pop are populations for year n+1

    // 3. Copy from next_fox_pop into fox_pop and
    //    from next_geese_pop into geese_pop.

    //Assert: Invariant is not true at this point.
    //Assert: fox_pop and geese_pop are correct populations for year n+1

    // 4. Increment the year
    ++n;
    //Invariant: fox_pop and geese_pop are correct populations for year n
    
}