Definition (RDF Literal). An RDF literal is either 1) a self-denoting plain literal of the form "<string>"(@, where <string> is a string and <lang> is an optional language tag, or 2) a typed literal of the form "<string>"^^<datatype>, where <datatype> is an IRI denoting a datatype according to a schema, such as the XML Schema, and <string> is an element of the lexical space corresponding to the datatype. A typed literal denotes the value derived from the lexical to value mapping of <datatype> to <string>. The typical annotation of the set of all literals is ?, which consists of the set ?P of plain literals and the set ?T of typed literals, respectively.