def index_map(letter):   
    """
    Defines the index of each 
    letter in the sequence
    :param letter: an a(A), 
    c(C), t(T), or g(G)
    :return: 0, 1, 2, 3
    """
    if letter == 'a' or \
    letter == 'A':
        return 0
    elif letter == 'c' or \
    letter ==  'C':
        return 1
    elif letter == 't' or \
    letter == 'T':
        return 2
    elif letter == 'g' or \
    letter == 'G':
        return 3
    else:
        return -1