How do I define new operators in Flop's custom_field?

I am building an API for querying spatio-temporal data and for filtering / sorting I chose to use Flop since it does most of what I need out of the box and most of what it doesn’t do out of the box, I can implement using custom_fields.

However, I don’t seem to get how to define new operators in custom_field’s. For example, I’d like to define Postgres’s range operators (<@, >@, &&, etc.) to use them in filters, but as soon as I try, it throws me an error and lists me the built-in operators.

For example, this doesn’t work:

@derive {
    Flop.Schema,
    filterable: [:id, :year_range],
    sortable: [:id],
    adapter_opts: [
      custom_fields: [
        year_range: [
          filter: {CustomFlopFilters, :year_range_filter, [source: :year]},
          ecto_type: Int4Range,
          operators: [:&&] # <-- Here is the problem
        ]
      ]
    ]
  }

Of course, I managed to get everything working by semantically abusing the built-in operators, e.g.:

@derive {
    Flop.Schema,
    filterable: [:id, :year_range],
    sortable: [:id],
    adapter_opts: [
      custom_fields: [
        year_range: [
          filter: {CustomFlopFilters, :year_range_filter, [source: :year]},
          ecto_type: Int4Range,
          operators: [:=~]
        ]
      ]
    ]
  }

But there comes a point where you need to define new ones, or maybe you prefer to use a notation that makes sense to you.

Any suggestions on how to proceed?