* An AST transform will take the schema and a document, and transform the AST * of the document returning the same document type * * With these transforms we can slightly change the GraphQL operation or * fragment compared to what the user supplies. This is helpful for instance * to add fields that clients will add implicitly, or to preprocess the document * to conform to some of the conventions of graphql-ppx.

* This is a GraphQL AST transform that will add the __typename to all selection * sets. This is necessary for Apollo, as this is an implicit field. * If we don't include this when using Apollo, the __typename information is * lost when we parse values and then serialize back to the js-values again

* This is a GraphQL AST transform that removes the `__typename` field from * unions. The PPX will add this in the printer stage, so it is always there. * The PPX will not allow any scalar fields on a union except for the inline * fragments, so we make sure to remove it before doing any further processing.