v5.9.1

ModelingToolkit v5.9.1

Diff since v5.9.0

Closed issues:

• Images for documentation (#833)
• Calculating observed variables which are functions of algebraic variables fails (#834)

Merged pull requests:

• Remove Symbolics.jl from MTK docs (#827) (@ChrisRackauckas)
• Add acausal modeling tutorial (#835) (@ChrisRackauckas)
• add a bunch of tutorials (#836) (@ChrisRackauckas)
• Handle dependencies of observed variables properly (#837) (@YingboMa)