  The $S$-matrix Ansatz for the construction of a quantum theory of black holes
is further exploited. We first note that treating the metric tensor $g_{\m\n}$
as an operator rather than a background allows us to use a setting where
information is not lost. But then we also observe that the 'trans-Planckian'
particles (particles with kinetic energies beyond the Planck energy) need to be
addressed. It is now postulated that they can be transformed into
'cis-Planckian particles' (having energies less that the Planck energy). This
requires the existence of a delicate algebra of operators defined at a black
hole horizon. Operators describing ingoing particles are mapped onto operators
describing outgoing ones, preserving their commutator algebra. At short
distance, the transverse gravitational back reaction dictates a discrete
lattice of data points, and at large distance the algebra must reproduce known
interactions of the Standard Model of elementary particles. It is suggested
that further elaboration of these ideas requiring complete agreement with
general relativity and unitarity should lead to severe restrictions concerning
the inter-particle interactions.
