You are given a text, which contains different English letters and punctuation marks.
You should find the most frequent letter in the text. The letter returned must be in lowercase.
During the search for the most wanted letter, casing doesn’t matter, "A" == "a". Make sure you don’t count in the punctuation marks, digits and whitespaces, just letters.
If you have two or more letters occurring the same number of times, then return all of them as a list. For example -- "Hello, Evan" should return ['e', 'l'].
Input: A text for analysis as a string.
Output: The list of the most frequent letters in lowercase.