Symmetry Scout

A signal sequence is considered a 'Symmetry Scout' if it reads the same forward and backward, but only when considering alphabetic characters. All digits, symbols, and spaces should be ignored, and letter casing should be treated as equal. Given a raw signal string, determine if it is a scout. Input Format: A single string of characters containing letters, numbers, and symbols. Output Format: Print 'YES' if it is a Symmetry Scout, otherwise print 'NO'. Example: Input: A1b@2a Output: YES (Explanation: Ignoring '1', '@', and '2', the sequence is 'Aba', which is a palindrome.)