st: margins with logarithmic offsets

["Dimitriy V. Masterov" <dvmaster@gmail.com>]

I have two questions about evaluating marginal effects at a specific
value of the logarithmic offset variable. Adding the at option to the
margins command gives offset not found in list of covariates error.

I did not find anything on this issue in the documentation, but there
was an old Stata FAQ about the mfx (an ancestor of margins) that
suggested merely replacing like this:

webuse dollhill3, clear
poisson deaths i.smokes i.agecat, exposure(pyears)
margins, dydx(smokes)
replace pyears =20000
margins, dydx(smokes)

1) Is there a less clunky way to do this?
2) Does this correspond to -margins, dydx(smokes) at(pyears ==20000)-?

Stata mfx FAQ link:
http://www.stata.com/support/faqs/statistics/marginal-effects-after-offset/
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