def count_occurrences(phrase: str, letter: str) -> int: """ Count occurrences of a letter in a phrase (case insensitive).

:param phrase: The phrase to search within.
:param letter: The letter to count occurrences of.
:return: The number of occurrences of the letter in the phrase.
"""
return phrase.lower().count(letter.lower())

Test cases

print(count_occurrences("letter", "t")) # Output: 2 print(count_occurrences("abc", "a")) # Output: 1 print(count_occurrences("abc", "d")) # Output: 0 print(count_occurrences("ABC", "a")) # Output: 1