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NI is used in a few special idiomatic number phrases:

                  +--NId--+   |
                  +-NIa+  |   +D-+
	          |    |  |   |  |
	I have between 5 and 20 dogs
	*I have between dogs and cats

The first word of the expression basically connects all the
words together; but the last word of the expression (a number)
connects the phrase to the outside world. 

NIa is also used in a few idiomatic number expressions: "He is
aged 70", "He is on flight 714". It is used, as well, for the
dollar sign "$". This symbol must make an NIa connection
forward to a number; it may then make act as a noun phrase, 
making an S, O, AN, or J connection.

         +---O--+               +--AN---+
         |   +NI+               +NI+
         |   |  |               |  |    |
      I have $ 30, but I want a $ 5000 car

Grammar Documentation Page.