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+# Model wrappers
+
+A model that can have one or more other models as hyper-parameters should overload the trait `is_wrapper`, as in this example:
+
+```julia
+MLJModelInterface.target_in_fit(::Type{<:MyWrapper}) = true
+```
+
+The constructor for such a model does not need provide default values for the model-valued
+hyper-parameters. If only a single model is wrapped, then the hyper-parameter should have
+the name `:model` and this should be an optional positional argument, as well as a keyword
+argument.
+
+For example, `EnsembleModel` is a model wrapper, and we can construct an instance like this:
+
+```julia
+using MLJ
+atom = ConstantClassfier()
+EnsembleModel(tree, n=100)
+```
+
+but also like this:
+
+```julia
+EnsembleModel(model=tree, n=100)
+```
+
+This is the only case in MLJ where positional arguments in a model constructor are
+allowed.