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X<br />
α β<br />
A<br />
First(x)函数 —— 实现Lookahead<br />
*<br />
First(x) = {α | x ⇒αβ, α∈VT , β∈VN X<br />
α β<br />
First(x) = {x} 若 x ∈ V T<br />
示例:<br />
(1) S NP VP<br />
(2) NP N<br />
(3) NP CS 的<br />
(4) VP V NP<br />
(5) CS NP V '<br />
(6) V ' V V<br />
U<br />
V T }<br />
终结符的first集是它自身<br />
First(S) ={N}<br />
First(NP) = {N}<br />
First(N) = {N}<br />
First(CS) = {N}<br />
First(VP)={V}<br />
First(V ') = {V}<br />
4