~.  (Verb)

NUB (_)  ~.y selects the nub of y, that is, all of its distinct items. For example:

	  y	        ~. y	
ABC       ABC
ABC       DEF
DEF
   ~.3       $~. 3
3         1

More precisely, the nub is obtained by selecting the leading item, suppressing from the argument all items tolerantly equal to it, selecting the leading remaining item, and so on.