Write a function that takes a string as its only argument and returns a sorted list of characters used in the string. The resulting list should only contain lower case letters and each letter should...

Extracted text: Write a function that takes a string as its only argument and returns a sorted list of characters used in the string. The resulting list should only contain lower case letters and each letter should exist only once. Also, the resulting list shouldn't contain any spaces or punctuation marks, such as ",;.!?". For example, strletters("We are, the Champions!") returns ['a', 'c', 'e', 'h', 'i', 'm', 'n', 'o', 'p', 'r', 's', 't', 'w'] def strletters(istr): return # Remove this line to answer this question.