A list of formulas that describe the model. For simple models,
only one list-element, conditional, is returned. For more complex
models, the returned list may have following elements:
conditional, the "fixed effects" part from the model
random, the "random effects" part from the model (or the id for gee-models and similar)
zero_inflated, the "fixed effects" part from the zero-inflation component of the model
zero_inflated_random, the "random effects" part from the zero-inflation component of the model
dispersion, the dispersion formula
instruments, for fixed-effects regressions like ivreg, felm or plm, the instrumental variables
cluster, for fixed-effects regressions like felm, the cluster specification
correlation, for models with correlation-component like gls, the formula that describes the correlation structure
slopes, for fixed-effects individual-slope models like feis, the formula for the slope parameters