2. PRESIDENTIAL DOCUMENT IDENTIFIERS (PDI): Each distributed document carries
a DOCUMENT-ID header containing a PDI.  The document identifier is a
location-independent method of referring to documents. Various locations will
be able to resolve these document-ids and provide the document.  Currently,
there is a subject line command in the server to retrieve documents according
to their document-id.

We have developed a simple syntax for universal document identifiers that is
used in the DOCUMENT-ID field. The intention is for servers at different sites
to be able to return the document or information about the document based on
its PDI.

PDIs are compatible with URL parsers from WWW and have the following form:

White House example:	PDI://oma.eop.gov.us/1994/07/22/3.text.1

Generic:		PDI://document-series/year/month/day/number.format.version

Document-Series:	This is a hierarchical path, usually denoting the
			issuing authority but it definitely does not denote a
			physical address.

Year:			four digits.

Month:			two digits.

Day:			two digits.

Number:			the nth document issued under DOCUMENT-SERIES on DAY.

Format:			Indicates the desired format of the document (optional)
			We are currently supporting: text | summary

Version:		Integer indicating the preferred version (optional).  Relevant only
			when a document has been supersedes by a corrected version.