RDF.PrefixMap.add

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

add(prefix_map, prefix, namespace)

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Specs

add(t(), coercible_prefix(), coercible_namespace()) ::
  {:ok, t()} | {:error, String.t()}

Adds a prefix mapping to prefix_map.

Unless a mapping of prefix to a different namespace already exists, an :ok tuple is returned, otherwise an :error tuple.