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[lemma=exist] □
Enron , while a large and well-known player among energy trading entities in the United States , was one among many existing and potential new players in electricity and natural gas markets . In The Emotional Brain , Joseph LeDoux investigates the origins of human emotions and explains that many exist as part of complex neural systems that evolved to enable us to survive . Many existing computational models treat biological tissue as a continuum . The shift of population to coastal counties and the increased demand for coastal tourism destinations has exacerbated many existing beach erosion problems . Sharp economic downturns inevitably aggravate many existing environmental and social problems . Many existing multimedia systems provide a browsing-based user interface for their users .