def processHeader(f):
    """
    takes in a file handle, f, and reads off lines until it comes to the first
    alignment block line, or the first blank line. If blank it returns nothing,
    if an alignment line it returns that line.
    """
    for line in f:
        line = line.strip()
        if line == '':
            return None
        if line.startswith('a'):
            return line