6. The method according to claim 5 , wherein the input document is a structured document.
5. A computer-implemented method of applying abstraction by a validating parser, comprising: using, by a validating parser, a first schema definition for validating syntax elements when parsing syntax of an input document, wherein the first schema definition is identified in the input document; and omitting, by the validating parser when generating output from the parsed syntax of the input document, each of at least one of the validated syntax elements which is valid according to the first schema definition but is not valid according to a second schema definition for which the output is generated, wherein: the first schema definition is an extended schema; the second schema definition is a base schema from which the extended schema is extended, such that the extended schema defines at least one syntax element that is not defined in the base schema; and the second schema definition is dynamically requested, to the validating parser, by an application program for which the output is being generated.