def getPureOneBetweenAlphabet(trans, fromS, toS):
    """Function returned the set of alphabete used between the states
    "fromS" and "toS", without multipy uses (self loops) of the state "fromS".
    The alphabet is calculated only to the distance of one transition from
    the state "fromS".

    Args:
        trans (dict): Transtion dictionary.
        fromS (string): Source state.
        toS (string): Destination state.

    Returns:
        set: The set of the alphabet between states "fromS" and "toS" calculated
             on a distatnce of one transition.
    """

    alphabet = set()
    if fromS in trans:
        for letter in trans[fromS]:
            # Only transtions which leads to the "toS" counts.
            if toS in trans[fromS][letter]:
                alphabet.add(letter)
    
    # If the alphabet is empty (no transition leads from "fromS" to "toS"),
    # the set containing only None will be returned.
    if not alphabet:
        alphabet.add(None)
    
    return alphabet