/** XPath expression for extracting thumbnail content from meta tags */
    private static final String META_NAME_THUMBNAIL_CONTENT = "//META[@name=\"thumbnail\" or @name=\"THUMBNAIL\"]/@content";

    /** XPath expression for extracting Open Graph image content from meta tags */
    private static final String META_PROPERTY_OGIMAGE_CONTENT = "//META[@property=\"og:image\"]/@content";

# Maximum length of file title in documents.
crawler.document.file.max.title.length=100
# Maximum length of file digest in documents.
crawler.document.file.max.digest.length=200
# Whether to append meta content from files.
crawler.document.file.append.meta.content=true
# Whether to append body content from files.
crawler.document.file.append.body.content=true
# Default language for file documents.
crawler.document.file.default.lang=

    /* The iterator currently returning elements. */
    private Iterator<? extends T> iterator;

    /*
     * We track the "meta iterators," the iterators-of-iterators, below.  Usually, topMetaIterator
     * is the only one in use, but if we encounter nested concatenations, we start a deque of
     * meta-iterators rather than letting the nesting get arbitrarily deep.  This keeps each
     * operation O(1).
     */