Jan Lukasiewicz [http://en.wikipedia.org/wiki/Jan_Lukasiewicz]
was one of the mid-twentieth century's many luminescently
creative middle-European mathematicians.

This "Postfix Notation Mini-Lecture" [http://www.spsu.edu/cs/faculty/bbrown/web_lectures/postfix/]
explains some of his relevance to the theory of computing languages.
"PN" in this context can mean either "Postfix ..." or "Polish ..."  
It's possible that [Knuth] and HP, in different ways, 
had hands in popularizing the "Polish ..." reading.

See also: [Parsing Polish notation]

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