**An [isogram](https://en.wikipedia.org/wiki/Isogram)** (also known as a "nonpattern word") is a logological term for a word or phrase without a repeating letter. It is also used by some to mean a word or phrase in which each letter appears the same number of times, not necessarily just once.
You task is to write a method `isogram?` that takes a string argument and returns true if the string has the properties of being an isogram and false otherwise. Anything that is not a string is not an isogram (ints, nils, etc.)
**Properties:**
 - must be a string
 - cannot be nil or empty
 - each letter appears the same number of times (not necessarily just once)
 - letter case is not important (= case insensitive)
 - non-letter characters (e.g. hyphens) should be ignored
is_isogram = lambda word: type(word) == str and bool(word) and len(set( __import__("collections").Counter( __import__("re").sub(r'[^a-z]', "", word.lower())).values() )) == 1
is_isogram = lambda word: type(word) == str and bool(word) and len(set( __import__("collections").Counter( __import__("re").sub(r'[^a-z]', "", word.lower())).values() )) == 1