Write a recursive method to produce a pattern of n lines of asterisks. The first line contains one asterisk; the next line contains two, and so on, up to the nth line, which contains n asterisks. Line number n+1 again contains n asterisks; the next line has n-1 asterisks, and so on, until line number 2n, which has just one asterisk. Find the variant expressions and thresholds to show that this method never results in infinite recursion. Use induction to show that the method reach it's base case.