Given 2 strings, `a` and `b`, return a string of the form: `shorter+reverse(longer)+shorter.`
In other words, the shortest string has to be put as prefix and as suffix of the reverse of the longest.
Strings `a` and `b` may be empty, but not null (In C# strings may also be null. Treat them as if they are empty.).  
If `a` and `b` have the same length treat `a` as the longer producing `b+reverse(a)+b`
def shorter_reverse_longer(a,b):
  return a+b[::-1]+a if len(b)>len(a) else b+a[::-1]+b
def shorter_reverse_longer(a,b):
  return a+b[::-1]+a if len(b)>len(a) else b+a[::-1]+b