e and a list vars and returns a
lazy edist (expression distribution object), that is a subclass
of dist that can be used in place of a dist object.R/edist.R
    edist.RdTakes an expression e and a list vars and returns a
lazy edist (expression distribution object), that is a subclass
of dist that can be used in place of a dist object.
edist(e, vars)the expression to evaluate against the arguments.
the list of distributions (with variable names)
to evaluate the expression e against.
An edist object.