Reads letter aliases from a text file created by GoDepth1LettersWr.

def read_d1_letter(fin_txt):
    """Reads letter aliases from a text file created by GoDepth1LettersWr."""
    go2letter = {}
    re_goid = re.compile(r"(GO:\d{7})")
    with open(fin_txt) as ifstrm:
        for line in ifstrm:
            mtch = re_goid.search(line)
            if mtch and line[:1] != ' ':
                # Alias is expected to be the first character
                go2letter[mtch.group(1)] = line[:1]
    return go2letter