  To be compatible with general relativity, every fundamental theory should be
invariant under general coordinate transformations including spatial
reflection. This paper describes an extension of the standard model in which
the action is invariant under spatial reflection, and the vacuum spontaneously
breaks parity by giving a mean value to a pseudoscalar field. This field and
the scalar Higgs field make the gauge bosons, the known fermions, and a set of
mirror fermions suitably massive while avoiding flavor-changing neutral
currents. In the model, there is no strong-CP problem, there are no anomalies,
fermion number (quark-plus-lepton number) is conserved, and heavy mirror
fermions form heavy neutral mirror atoms which are dark-matter candidates. In
models with extended gauge groups, nucleons slowly decay into pions, leptons,
and neutrinos.
