Procedure to perform set differences between two lists. It returns a list of three lists: the first element returned is everything in the first list that isn't in the second list; the second element returned is the intersection of the two lists; the third element returned  is everything in the second list that isn't in the first list. The returned lists will be sorted.

Usage:
  intersect3 lista listb

Example:
  package require Tclx
  intersect3 {a b c d e} {d e d f g c} ;# ==> {a b} {c d e} {f g}

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[Category Command], part of [Tclx]