def get_symbols(wordslist):
    """
    Collect the set of letters from a list of strings.
    """
    symbols = []
    for word in wordslist:
        for x in word:
            x = x.lower()
            if not x.isalpha():
                raise ValueError("Only a-z and A-Z are allowed.")
            if x not in symbols:
                symbols.append(x)
    return sorted(symbols)