Methods used to parse the query string may differ between websites. Implementations of DFDL processors that can parse and serialize data using DFDL schemas are available. According to the Picasa Readme, Picasa can parse Extensible Metadata Platform (XMP) data. Web browsers rely on layout engines to parse HTML into a DOM. The browser may choose to first display the text, which it can parse the quickest. This gives a very helpful error message and resynchronizes the parse well. Prolog is used for pattern matching over natural language parse trees. They ignore most of the parse stack when making decisions. A head-driven phrase structure grammar might parse this sentence differently. They are simpler on average than constituency-based parse trees because they contain fewer nodes. O functions could parse and print the P notation as well. None of the parse tree nodes numbered 7 and above exist yet. The Search Aggregator would then parse the results and display them in a user-friendly way. The difficulty is with understanding how English speakers parse such sentences. Some argue that PL/I is unusually hard to parse.