def open_txt(p):
    'Create sheet from `.txt` file at Path `p`, checking whether it is TSV.'
    with p.open_text() as fp:
        if options.delimiter in next(fp):    # peek at the first line
            return open_tsv(p)  # TSV often have .txt extension
        return TextSheet(p.name, p)
Create sheet from `.txt` file at Path `p`, checking whether it is TSV.