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：
, O+ ]& T7 m! g5 I/ E7 J8 R7 `1 knet=newff([-1 2; 0 5],[3,1],{'tansig','purelin'},'traingd');
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net.trainParam.show = 50;
8 r2 U' p( ?0 i/ f4 Onet.trainParam.lr = 0.05;
net.trainParam.epochs = 300;
& W' ?9 i8 p  ~* N2 n1 ^net.trainParam.goal = 1e-5;
" E" Y$ v5 o" x* s5 G/ ^) V4 ]0 Sp = [-1 -1 2 2;0 5 0 5];
t = [-1 -1 1 1];
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7 ^" m0 X4 E! jnet=train(net,p,t);
TRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10


TRAINGD, Epoch 50/300, MSE 0.00236382/1e-05, Gradient 0.0495292/1e-10
# U4 [; f' M1 o8 c9 k+ G" LTRAINGD, Epoch 100/300, MSE 0.000435947/1e-05, Gradient 0.0161202/1e-10
TRAINGD, Epoch 150/300, MSE 8.68462e-05/1e-05, Gradient 0.00769588/1e-10
TRAINGD, 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
TRAINGD, Performance goal met.
8 k4 u3 K" _0 Y8 r8 f! t: m" Ia = sim(net,p)
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plot(p,t,p,a,'o')
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, G# U% l. S4 t但是为什么老是报错：
??? Input argument "net" is undefined.