# fit_members errors informatively with a bad model_stack arg

    Code
      st_reg_1 %>% fit_members()
    Condition
      Error in `fit_members()`:
      ! The supplied `model_stack` argument is a data stack rather than a model stack. Did you forget to first evaluate the ensemble's stacking coefficients with `blend_predictions()` (`?stacks::blend_predictions()`)?

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    Code
      "howdy" %>% fit_members()
    Condition
      Error in `fit_members()`:
      ! Element model_stack needs to inherit from `model_stack`, but its class is `character`.

---

    Code
      out <- st_reg_1__ %>% fit_members()
    Condition
      Warning:
      The members in the supplied `model_stack` have already been fitted and need not be fitted again.

# fit_members checks for required packages

    The package a needs to be installed before fitting members.

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    The packages a and b need to be installed before fitting members.

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    The packages a, b, and c need to be installed before fitting members.

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    The packages recipes, parsnip, and kernlab need to be installed before fitting members.

