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：
( }; z/ k: T! R3 y/ ~net=newff([-1 2; 0 5],[3,1],{'tansig','purelin'},'traingd');


net.trainParam.show = 50;
  V! ^! ]; R  R8 ~7 j  Znet.trainParam.lr = 0.05;
net.trainParam.epochs = 300;
net.trainParam.goal = 1e-5;
p = [-1 -1 2 2;0 5 0 5];
% [7 Q/ K' N$ Vt = [-1 -1 1 1];
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4 L0 f( l+ m( Nnet=train(net,p,t);
  R( g# H# Y6 L. T7 H( a: u  `, ETRAINGD, Epoch 0/300, MSE 1.59423/1e-05, Gradient 2.76799/1e-10
- V* {* T; B  D& `
, Z1 |2 ^7 E+ {1 {
  P1 ~+ W, O# X! F( O2 ATRAINGD, 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
: b: H! j- k9 C7 ^" |4 STRAINGD, Epoch 150/300, MSE 8.68462e-05/1e-05, Gradient 0.00769588/1e-10
8 v( f8 l% {# {' r3 aTRAINGD, 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
. C; o0 n' b6 L2 L7 }6 ?5 a: `; w% ETRAINGD, Performance goal met.
3 s* e( q6 Y0 [) ~# c% ia = sim(net,p)
: R' _; {* {8 ~1 S7 p

: u1 T5 p, n4 t7 Z, U6 Eplot(p,t,p,a,'o')
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但是为什么老是报错：
+ a( h4 D. K) ]: a( ^' ^1 G??? Input argument "net" is undefined.