函数下面的（著名的 Zilberstein）PR（uning）算法是如何LP-dominate开始的：第一次调用它，D=∅线性程序恶化（即没有约束方程）？

procedure POINTWISE-DOMINATE(w, U)
...
3. return false
procedure LP-DOMINATE(w, U)
4. solve the following linear program variables: d, b(s) ∀s ∈ S
      maximize d
      subject to the constraints
        b · (w − u) ≥ d, ∀u ∈ U
        sum(b) = 1
5. if d ≥ 0 then return b
6. else return nil
procedure BEST(b, U )
...
12. return w
procedure PR(W)
13. D ← ∅
14. while W = ∅
15.   w ← any element in W
16.   if POINTWISE-DOMINATE(w, D) = true
17.      W ← W − {w}
18.   else
19.      b ← LP-DOMINATE(w, D)
20.      if b = nil then
21.         W ← W − {w}
22.      else
23.         w ← BEST(b, W)
24.         D ← D ∪ {w}
25.         W ← W − {w}
26. return D