I have the following data

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input int(year exporter importer) float imports
2003 757 788  121.003
2003 757 788   19.121
2003 757 788   14.156
2003 757 788   12.687
2003 757 788  256.345
2003 757 788  286.039
2003 757 788   62.286
2003 757 788    6.842
2003 757 788     .368
2003 757 788     .135
2003 757 788   27.257
2003 757 788    1.357
2003 757 788    2.856
2003 757 788    4.181
2003 757 788    1.221
2003 757 788   20.043
2003 757 788    103.2
2003 757 788    7.064
2003 757 788   76.597
2003 757 788  241.152
2003 757 788   35.909
2003 757 788   52.241
2003 757 788    7.811
2003 757 788    64.57
2003 757 788   76.173
2003 757 788    5.565
2003 757 788  180.232
2003 757 788   43.568
2003 757 788    7.251
2003 757 788 1432.262
2003 757 788     .781
2003 757 788    3.926
2003 757 788  162.957
2003 757 788   26.317
2003 757 788  449.058
2003 757 788    1.919
2003 757 788     .117
2003 757 788     .195
2003 757 788      .03
2003 757 788    1.408
2003 757 788    49.54
2003 757 788   15.562
2003 757 788     .269
2003 757 788     .073
2003 757 788     .032
2003 757 788     .017
2003 757 788    4.767
2003 757 788   10.516
2003 757 788    7.098
2003 757 788    1.595
2003 757 788    8.878
2003 757 788    1.438
2003 757 788    2.055
2003 757 788    17.73
2003 757 788    4.324
2003 757 788   15.175
2003 757 788    2.603
2003 757 788   13.217
2003 757 788    1.041
2003 757 788    3.158
2003 757 788    1.175
2003 757 788    3.583
2003 757 788   18.869
2003 757 788     .372
2003 757 788     .389
2003 757 788    2.713
2003 757 788    3.971
2003 757 788   37.492
2003 757 788    2.082
2003 757 788      .29
2003 757 788  112.364
2003 757 788     2.46
2003 757 788     .538
2003 757 788    15.45
2003 757 788    6.821
2003 757 788      .64
2003 757 788    4.042
2003 757 788    4.319
2003 757 788     .959
200 757 788    1.939
2003 757 788    4.116
2003 757 788     .058
2003 757 788    5.119
2003 757 788    4.012
2003 757 788  199.547
2003 757 788   71.131
2003 757 788   16.162
2003 757 788   11.267
2003 757 788    1.862
2003 757 792     .569
2003 757 792   23.169
2004 757 792   18.307
2004 757 792  490.402
2004 757 792   75.016
2004 757 792     .318
2004 757 792     5.27
2004 757 792    6.708
2004 757 792    23.69
2004 757 792    173.7
2004 757 792    33.36
end
Here is my command:

collapse（sum）imports if exporter==importer, by (year)

But it shows that the command collapse（sum）imports is unrecognized
I can not figure out the reason.
Thanks in advance!