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NS serves a similar function to ND, only for singular expressions.
(See "ND".) The words "one", "1", and "a (an)" therefore carry "NS+";
singular words like "week", used in idiomatic number expressions,
carry NS-. 

	          |   |   
	I saw him a week ago

A few expressions of the form "a [noun]" can act like
prepositional phrases: "You should eat three meals A DAY", "It
costs five dollars A POUND". For this, only "a" can be used,
not "1". Idiomatic dictionary entries for "day" and similar
words are given "NSa"; "1" and "one" are given "NSn".

Grammar Documentation Page.