Hi. Excuse me for poor-quality English language.
I was interested by an opportunity of use of the operator "UnsortedComplement" for subtraction previously of sorted lists without program sorting, which is carried out by one pass.
The purpose: reduction of an operating time of the program.
Beforehand I want to thank you for the answer.
(* Moderator's Note: Unsorted Complement is a user
defined function described in the documentation,
see: http://documents.wolfram.com/v4/RefGuide/Complement_ex.html *)