  Let H be a bialgebra and D an H-bimodule algebra H-bicomodule coalgebra. We
find sufficient conditions on D for the L-R-smash product algebra and coalgebra
structures on D\otimes H to form a bialgebra (in this case we say that (H, D)
is an L-R-admissible pair), called L-R-smash biproduct. The Radford biproduct
is a particular case, and so is, up to isomorphism, a double biproduct with
trivial pairing. We construct a prebraided monoidal category {\cal LR}(H),
whose objects are H-bimodules H-bicomodules M endowed with left-left and
right-right Yetter-Drinfeld module as well as left-right and right-left Long
module structures over H, with the property that, if (H, D) is an
L-R-admissible pair, then D is a bialgebra in {\cal LR}(H).
