How to write variations in mod file

Hello, i have a little problem with mod file. i was wondering how to write " ∆m ̃_(t-1)" with dynare?

∆m ̃_{t-1} = m ̃_{t-1} - m ̃_{t-2} ?? in your model?

in my model : ∆ ln mt = ln mt − ln mt−1

add dm to var block, and define dm = m - m(-1) in model block.

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Thanks for your help :slight_smile: