  We study the Eisenstein series and constant term functors in the framework of
geometric theory of automorphic functions. Our main result says that for a
parabolic P in G with Levi quotient M, the !-constant term functor
CT_!:D-mod(Bun_G)-> D-mod(Bun_M) is canonically isomorphic to the *-constant
term functor CT^-_*:D-mod(Bun_G)\to D-mod(Bun_M), taken with respect to the
opposite parabolic P^-.
