As documented at https://github.com/folio-org/mod-graphql/blob/master/create-schemas/README.md#configuration-file-format the create-schemas script can overlay virtual fields into the raw schemas that it downloads. This makes it possible to configure, for example, a virtual field for holdings within an instance:

However, this expands to an overlay includes a single record as result of the search instanceId=id. That's OK for virtual fields of the form "the instance that this holdings record pertains to", but not for the case above, which is "the set of holdings records that pertain to this instance".

We need syntax to have create-schemas make an array-types field.