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Calculation routines based on the FOCUS Kinetics Report (2006, 2014). Includes a function for conveniently defining differential equation models, model solution based on eigenvalues if possible or using numerical solvers. If a C compiler (on windows: 'Rtools') is installed, differential equation models are solved using automatically generated C functions. Heteroscedasticity can be taken into account using variance by variable or two-component error models as described by Ranke and Meinecke (2018, doi:10.3390/environments6120124). Hierarchical degradation models can be fitted using nonlinear mixed-effects model packages as a back end as described by Ranke et al. (2021, doi:10.3390/environments8080071). Please note that no warranty is implied for correctness of results or fitness for a particular purpose.