st: How to generate lagged variables

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I have an unbalanced panel with 100 firms, 4 years, and about 300 obs.
Some firms provide information for all empirical years, while others provide
for less than 4 periods.
Using this panel, I need to create lagged variables such as SALES(-1).
Which command in Stata7-SE should I use to do this?
Thank you in advance.
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