③苇大 计算 First集 To calculate First(u) where u has the form X1X2.Xn, do the following 1. If X1 is a terminal, then add X1 to First(u) otherwise add First(X1)-s to First(u) 2. If X1 is a nullable nonterminal. i e. x1=>c. add First(X2)-8 to First(u). Furthermore if X2 can also go to s, then add First(X3 )-8 and so on, through all xn until the first nonnullable one 3. If X1X2.Xn =>8. add s to the first set计算First 集 To calculate First(u) where u has the form X1X2...Xn, do the following: 1. If X1 is a terminal, then add X1 to First(u), otherwise add First(X1) -  to First(u ) . 2. If X1 is a nullable nonterminal, i.e., X1 =>*  , add First(X2) -  to First(u). Furthermore, if X2 can also go to  , then add First(X3) -  and so on, through all Xn until the first nonnullable one. 3. If X1X2...Xn =>*  , add  to the first set