# Changing the system size over time in ModellingToolkit

**URL:** <https://discourse.julialang.org/t/changing-the-system-size-over-time-in-modellingtoolkit/103100>\
**Category:** Modelling & Simulations\
**Created:** [August 23, 2023, 3:22am UTC](https://discourse.julialang.org/t/changing-the-system-size-over-time-in-modellingtoolkit/103100 "2023-08-23T03:22:27Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![evan-wehi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/evan-wehi/32/32692_2.png) [@evan-wehi](https://discourse.julialang.org/u/evan-wehi)\
**Post date:** [August 23, 2023, 3:22am UTC](https://discourse.julialang.org/t/changing-the-system-size-over-time-in-modellingtoolkit/103100/1 "2023-08-23T03:22:27Z")

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I apologise if this is answered in the documentation but I couldn’t find it.

I’m modelling a population of biological cells which divide and die over time. These events can be triggered by continious callbacks for example. The DifferentialEquations package has an API that allows the system to dynamically add and remove state vectors from the problem in the `afftect!` callback that is perfect for modelling cell birth and death. Is there something a corresponding API for adding removing equations in ModellingToolkit?

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**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [August 23, 2023, 3:58am UTC](https://discourse.julialang.org/t/changing-the-system-size-over-time-in-modellingtoolkit/103100/2 "2023-08-23T03:58:52Z")

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Not yet.

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**Author:** ![sabbraxcaddabra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sabbraxcaddabra/32/51312_2.png) [@sabbraxcaddabra](https://discourse.julialang.org/u/sabbraxcaddabra)\
**Post date:** [August 25, 2023, 10:15pm UTC](https://discourse.julialang.org/t/changing-the-system-size-over-time-in-modellingtoolkit/103100/3 "2023-08-25T22:15:58Z")

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Hello, are there any plans to implement this?

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**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [August 26, 2023, 8:06am UTC](https://discourse.julialang.org/t/changing-the-system-size-over-time-in-modellingtoolkit/103100/4 "2023-08-26T08:06:04Z")

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Yes, but not necessarily soon. It’s not an easy one