  We introduce and study a Fock-space noncommutative analogue of reproducing
kernel Hilbert spaces of de Branges-Rovnyak type. Results include: use of the
de Branges-Rovnyak space ${\mathcal H}(K_{S})$ as the state space for the
unique (up to unitary equivalence) observable, coisometric transfer-function
realization of the Schur-class multiplier $S$, realization-theoretic
characterization of inner Schur-class multipliers, and a calculus for obtaining
a realization for an inner multiplier with prescribed left zero-structure. In
contrast with the parallel theory for the Arveson space on the unit ball
${\mathbb B}^{d} \subset {\mathbb C}^{d}$ (which can be viewed as the
symmetrized version of the Fock space used here), the results here are much
more in line with the classical univariate case, with the extra ingredient of
the existence of all results having both a ``left'' and a ``right'' version.
