新手求助~（关于初级的神经网络问题）

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各位大虾，我在网上看到的教程里的一个程序段，本来想要试试看的：
# F+ H6 \+ L- O* i: o. {net=newff([-1 2; 0 5],[3,1],{'tansig','purelin'},'traingd');

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9 A1 A7 Z# T- r# i+ Tnet.trainParam.show = 50;
net.trainParam.lr = 0.05;
net.trainParam.epochs = 300;
! v# C' ]  x, ]$ ~7 Y  `! q. ~net.trainParam.goal = 1e-5;
" P4 c9 b5 p0 k! v) {. j. D7 ?p = [-1 -1 2 2;0 5 0 5];
t = [-1 -1 1 1];
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- g& d4 K0 w) }: t- Inet=train(net,p,t);
- h) p$ i6 W: QTRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10

/ K3 k/ L7 a! ]* R- Z' C
TRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
TRAINGD, Epoch 100/300, MSE 0.000435947/1e-05, Gradient 0.0161202/1e-10
( U4 u* f/ l- A6 z7 K+ N  {6 CTRAINGD, Epoch 150/300, MSE 8.68462e-05/1e-05, Gradient 0.00769588/1e-10
0 [+ h$ D: u- }* U- ?* v! nTRAINGD, Epoch 200/300, MSE 1.45042e-05/1e-05, Gradient 0.00325667/1e-10
TRAINGD, Epoch 211/300, MSE 9.64816e-06/1e-05, Gradient 0.00266775/1e-10
) {% Z9 e# T2 J: ?9 Y: LTRAINGD, Performance goal met.
' E4 D! w% J, o& Ia = sim(net,p)

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plot(p,t,p,a,'o')
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但是为什么老是报错：
??? Input argument "net" is undefined.