5eebb559_nd_for_the_PostScript_language__Purpose
[Key] PartLine [Purpose] 
Procedure to be executed when readline has returned false, meaning it encountered the end of input (or an interruption for embedded PostScript) before a newline. The line may be of zero or nonzero length. The stack contains (a) whatever was on it when \markup was invoked (except for the parameter dictionary), (b) as modified by any embedded PostScript tokens since, and not consumed by HandleToken or HandleResult, and (c) on top, the string just read. \markup fetches this value only once, so changing it from embedded PostScript code has no effect, but nothing stops the supplied procedure looking at other keys and having changeable behavior.
Markup: a minimalist markup frontend for the PostScript language
Purpose