Write a Python function that returns out a set of letters appeared in the sentence.

OUTPUT :

CODE:
def letters(s):
#Set to store ans
ans = set()
#Loop through all chars in sentence
for c in s:
#Check if char is letter
if c.isalpha():
#Add letter to set
ans.add(c)
#Return ans
return ans
Write a Python function that returns out a set of letters appeared in the sentence.