index on eExpression tag mytag
*--- where eExpression is the field or expression
*--- on which you want to index
...
...
seek(eExpression)
*--- where eExpression is the expression to be sought in the index.
*--- SEEK() is Rushmore-optimized so it will find your record VERY quickly.

*--- You can also use LOCATE FOR and SCAN...ENDSCAN whether the cursor is indexed or not.
*--- Unless it is a huge cursor you can often make do with a LOCATE, without an index.