? on "Eval" function in operational semantics of PRS (RIF-PRD)

Hi,

I can not follow the definition of the Eval function in section [1]:

Given a rule set RS ⊆ R, the associated conflict resolution strategy, LS,
and halting test, H, and an initial state of the fact base, w ∈ W, the input
function to a RIF-PRD production rule
system<http://www.w3.org/TR/rif-prd/#def-prs> is
defined as:
*Eval(RS, LS, H, w) →PRS s ∈ S*, such that facts(s) = w and previous(s) =
NIL.
Although, *Eval(RS, LS, H, w) →*PRS s'* gives some sense of what Eval(RS,
LS, H, w) is, I am confused by : *Eval(RS, LS, H, w) →PRS s ∈ S*. Can you
please explain?
[1]
http://www.w3.org/TR/rif-prd/#Operational_semantics_of_a_production_rule_system
thanks,ankesh