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	This polymorphic function produces an error if the input value is detected as being the null value; otherwise it returns the input value unchanged. This function can for example become handy in SQL function bodies, to enforce that exactly one row was returned. Author: Joel Jacobson <joel@compiler.org> Reviewed-by: Vik Fearing <vik@postgresfriends.org> Reviewed-by: Michael Paquier <michael@paquier.xyz> Discussion: https://postgr.es/m/ece8c6d1-2ab1-45d5-ba12-8dec96fc8886@app.fastmail.com Discussion: https://postgr.es/m/de94808d-ed58-4536-9e28-e79b09a534c7@app.fastmail.com
<!-- doc/src/sgml/README.non-ASCII -->
Representation of non-ASCII characters
--------------------------------------
Find non-ASCII characters using:
        grep --recursive --color='auto' -P '[\x80-\xFF]' .
Convert to HTML4 named entity (&) escapes
-----------------------------------------
We support several output formats:
*  html (supports all Unicode characters)
*  man (supports all Unicode characters)
*  pdf (supports only Latin-1 characters)
*  info
While some output formatting tools support all Unicode characters,
others only support Latin-1 characters.  Specifically, the PDF rendering
engine can only display Latin-1 characters;  non-Latin-1 Unicode
characters are displayed as "###".
Therefore, in the SGML files, we can only use Latin-1 characters.  We
can use UTF8 representations of Latin-1 characters, or HTML entities of
Latin-1 characters, e.g., Álvaro.
Do not use UTF numeric character escapes (&#nnn;).
When building the PDF docs, problem characters will appear as warnings.
HTML entities
        official:      http://www.w3.org/TR/html4/sgml/entities.html
        one page:      http://www.zipcon.net/~swhite/docs/computers/browsers/entities_page.html
        other lists:   http://www.zipcon.net/~swhite/docs/computers/browsers/entities.html
                       http://www.zipcon.net/~swhite/docs/computers/browsers/entities_page.html
                       https://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references