Write a function called `is_valid_isbn(astring)` that takes the string entitled astring and returns the Boolean result of checking if astring is a valid ISBN (International Standard Book Number) number. The parameter astring has two common formats. One is a-bcde-fghi-j, and the other is abcdefghij. The first digit a denotes the country where the book is published or the language of the book. The digits bcde identify the publisher, the digits fghi the book, and the digit j represents the check digit. It value is such that

(10*a + 9*b + 8*c + 7*d + 6*e + 5*f + 4*g + 3*h + 2*i + j) mod 11 = 0

The character X is used for the digit j if j must be 10.

In python.

def is_valid_isbn(astring):