  We examine flat models containing a dark matter component and an arbitrary
dark energy component, subject only to the constraint that the dark energy
satisfies the weak energy condition. We determine the constraints that these
conditions place on the evolution of the Hubble parameter with redshift, H(z),
and on the scaling of the coordinate distance with redshift, r(z).
Observational constraints on H(z) are used to derive an upper bound on the
current matter density. We demonstrate how the weak energy condition constrains
fitting functions for r(z).
