Thanks, this is nice. I presume this comes from fluids? I found this wiki link: https://en.wikipedia.org/wiki/Lagrangian_and_Eulerian_specification_of_the_flow_

Thanks, this is nice. I presume this comes from fluids? I found this wiki link: https://en.wikipedia.org/wiki/Lagrangian_and_Eulerian_specification_of_the_flow_field https://en.wikipedia.org/wiki/Lagrangian_and_Eulerian_specif.... Is this it?