RDF.Data.merge

You're seeing just the function merge, go back to RDF.Data module for more information.
Link to this function

merge(data, input, opts \\ [])

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Adds statements to a RDF data structure.

As opposed to the specific add functions on the RDF data structures, which always return the same structure type than the first argument, merge might result in another RDF data structure, eg. merging two RDF.Description with different subjects results in a RDF.Graph or adding a quad to a RDF.Graph with a different name than the graph context of the quad results in a RDF.Dataset. But it is always guaranteed that the resulting structure has a RDF.Data implementation.