def add_single_filter(aOutput, iTocIndex, sKeyword, oFilter):
    """Add a single filter's help text to the file"""
    sDesc = oFilter.description
    sLink = sKeyword.lower()
    aOutput.insert(iTocIndex, '## "%s":#%s\n' % (sDesc, sLink))
    iTocIndex += 1
    aOutput.append('h3(#%s). %s\n\n' % (sLink, sDesc))
    try:
        sInput, sRest = oFilter.helptext.split('\n', 1)
        aOutput.append('*Parameters:* %s\n\n' % sInput)
        aOutput.append(sRest)
    except ValueError:
        print('Failed to extract filter details')
        print(oFilter.keyword, oFilter.helptext)
    aOutput.append('\n\n')
    return iTocIndex
