杰弗瑞。杰尼和菲利普。瑞尼的高微，博弈论定理7.1，看不懂，求高手指点。。THEOREM 7.1 Simplified Nash Equilibrium Tests
The following statements are equivalent:
(a) mˆ ∈ M is a Nash equilibrium.
(b) For every player i, ui(mˆ ) = ui(si,mˆ −i) for every si ∈ Si given positive weight by
mˆ i, and ui(mˆ ) ≥ ui(si,mˆ −i) for every si ∈ Si given zero weight by mˆ i.
(c) For every player i, ui(mˆ ) ≥ ui(si,mˆ −i) for every si ∈ Si.
positive weight是什么意思啊，zero weight又是什么意思；后面的例题说“运用定理（b)部分每个局中人必定会对其每种纯对策无差异”这句话又是怎么理解？