Set Empty
Definition: Let G = (N, T, P, S) be a CFG.
Empty(x) = {e} if x Þ* e; otherwise,
Empty(x) = Æ, where x Î (N È T)*.
 Gist:
Empty(x) is the set that include e if x derives the empty string; otherwise, Empty(x) is empty
Illustration:
x =
X1
X2
Xn
…
x = X1X2 …Xn Þ* e
Empty(x) = {e}
e
e
e
…
 e
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