def stringfy(words, start, end):
    """
    Converts a sentence with an entity marked by start and end into a single
    string.
    """
    string = ""
    delim = "__"
    start_marker = "{{"
    end_marker = "}}"
    for i, word in enumerate(words):
        if i == start: word = start_marker + word
        if i == end: word = word + end_marker
        if i < len(words) - 1: word = word + delim
        string += word
    return string