def decrypt_letter(encrypted_letter, keystream):
    """ (str, int) -> str
    
    Translate the encrypted_letter to a number in the range 0-25.  
    'A' translates to 0, 'B' to 1, 'Z' to 25, and so on.    
    Subtract the keystream from the number, modulo 26, convert back to a letter, 
    and return the unencrypted result.
    
    >>> decrypt_letter('X', 12)
    'L'
    >>> decrypt_letter('I', 1)
    'H'
    """
    
    # Translate to a number in the range 0-25.  'A' translates to 0, 'B' to 1,
    # and so on.
    ord_diff = ord(encrypted_letter) - ord('A')
    
    # Apply the keystream; we use % to handle the end of the alphabet.
    # The result is still in the range 0-25.
    new_char_ord = (ord_diff - keystream) % 26
        
    # Convert back to a letter.
    return chr(new_char_ord + ord('A'))