我想计算一个指标是mretwd*上一年mretnd的，用stata怎么做？code是公司代码。它是一个时间序列数据。
code year trdmnt mretwd mretnd
2 2001 Aug .068881 .068691
2 2002 Feb .025819 .025819
2 2003 Aug .014129 .014129
2 2004 Oct -.077491 -.077491
2 2005 Jun .044165 .045161
2 2006 May -.041734 -.041734
2 2007 Jul .480126 .480126
2 2008 Mar .158042 .158042
2 2009 Jul .047843 .047843
2 2010 Oct .154762 .154762
4 2001 Oct -.124664 -.124664
4 2002 Aug -.071683 -.071683
4 2003 Aug .006574 .006574
4 2004 Jun -.257919 -.257919
4 2005 Jan .012077 .012077
4 2006 May .4613 .4613
4 2007 Nov -.105731 -.105731
4 2008 Jan .182584 .182584
4 2009 Nov .119821 .119821
4 2010 Apr -.0478 -.0478
5 2001 Jul -.142385 -.142201
5 2002 Jul -.11437 -.11437
5 2003 Jun -.093834 -.093834
5 2004 Feb .07619 .07619
5 2005 Oct -.064935 -.064935
5 2006 Mar .433414 .433414
5 2007 Jul -.158427 -.158427
5 2008 Sep 0 0
5 2009 Aug -.239175 -.239175
5 2010 Apr -.046914 -.046914