Checks if `mod` argument is valid. This argument must be either `"1"` (indicating an intercept-only model) or a moderator that is included in the meta-analytic model (as specified in `model$formula.mods`).
mod_is_valid(model, mod)A logical value: TRUE if `mod` is valid, strop and throw an error otherwise.
A meta-analytic model from the metafor package.
A string specifying the moderator to check. Must be `"1"` or one of
the moderators included in model$formula.mods.