Letter Frequency

From ECOO 2011 https://ecoocs.org/contests/ecoo_2011.pdf

It has been said that the most common letter in the English alphabet is the letter "E". In actual messages that is not always the case. In this program you are asked to write a program that will find the most common letter in a message, ignoring upper case or lower case, not counting any non‐ alphabetic characters such as spaces, commas or numbers. State the number of occurrences of the letter, and if there is a tie, list each letter in the tie in alphabetic order.

Program Modification

Use dictionaries to track the frequency of each letter.

Sample Input

Beware the Jabberwock, my son!

the jaws that bite,

and the claws that catch!

Beware the Jujub bird, and shun

This frumious Bandersnatch!

Sample Output (not the actual output)

A E T occur(s) 4 times.

B E S U occur(s) 3 times.

F occur(s) 3 times.