Instead of positive? use (>/c 0).
hmc: Oops.  Record the DB from the first successful run.

Otherwise it was just rejection sampling.
hmc: Oops.  Record the DB from the first successful run.

Otherwise it was just rejection sampling.
Merge branch 'wip/hmc-sampler' into wip/hmc-label-derivs

Conflicts:
	prob/private/prob-hmc.rkt
Add dist-param-count query for distributions.
Initial implementation of hmc-gradient-potential-fn

Compute derivatives of distribution samples with respect to time using
parameter derivative annotations.

TODO #1: Memoize repeated computation of dependent random variable
      derivatives.
TODO #2: Documentation and testing.