def findImplicitInputFile():
    """
    Find what input file the user *probably* wants to use,
    by scanning the current folder.
    In preference order:
    1. index.bs
    2. Overview.bs
    3. the first file with a .bs extension
    4. the first file with a .src.html extension
    """
    import glob
    import os
    if os.path.isfile('index.bs'):
        return 'index.bs'
    if os.path.isfile('Overview.bs'):
        return 'Overview.bs'
    allBs = glob.glob('*.bs')
    if allBs:
        return allBs[0]
    allHtml = glob.glob('*.src.html')
    if allHtml:
        return allHtml[0]
    return None
