def encrypt_letter(unencrypted_letter, keystream):
    """ (str, int) -> str
    
    Translate the unencrypted_letter to a number in the range 0-25.  
    'A' translates to 0, 'B' to 1, 'Z' to 25, and so on.    
    Add the number with the keystream, modulo 26, convert back to a letter, 
    and return the encrypted result.
    
    >>> encrypt_letter('L', 12)
    'X'
    >>> encrypt_letter('H', 1)
    'I'
    """
    
    # Translate to a number in the range 0-25.  'A' translates to 0, 'B' to 1,
    # and so on.
    ord_diff = ord(unencrypted_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'))