Empty(X) for Gexpr3: Example
Empty( i ) := Æ
Empty( + ) := Æ
Empty( * ) := Æ
Empty( ( ) := Æ
Empty( ) ) := Æ
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Initialization:
Empty(E) :=  Æ
Empty(E’) := {e}
Empty(T) :=  Æ
Empty(T’) := {e}
Empty(F) :=  Æ
• No Empty set can be changed.
Gexpr3 = (N, T, P, E), where: N = {E, F, T}, T = {i, +, *, (, )},
P = { 1: E  ® TE’,   2: E’ ® +TE’, 3: E’ ® e,    4: T  ® FT’
5: T’ ® *FT’, 6: T’ ® e,        7: F  ® (E), 8: F  ® i }