Lag/Lead Operators in @formula

[junder873]

Maybe this is an issue with StatsModels.jl, but I was wondering if there is a plan to allow lag and lead operators in formula (similar to R fixest). Originally I was going to post this in StatsModels.jl, since it doesn't seem like any Julia regression package implements lag/lead right now, but I saw this pull request that seemed to suggest it was in this package.

[matthieugomez]

I want to wait until Julia has a better lag function
JuliaArrays/ShiftedArrays.jl#37
For now, simply create the variable before generating the formula.

[eloualiche]

Closing this as lead/lag should be implemented in a separate package.
This outside of the scope of fixedeffectmodels.

We have posted on shiftedarrays for this feature; hopefully we will find a way to implement it in the future.