There are but a handful of well-known improvisers who call the cello their main instrument: Hank Roberts, Vincent Courtois, David Darling, Fred Katz and Erik Friedlander. Erik Friedlander has been steadily building an impressive body of work over the past ten years that has seen him emerge as the cellist on call for the New York downtown scene. Through recordings with artists that include Dave Douglas, Joe Lovano and John Zorn, and now with his own groups, Friedlander is helping to redefi …