How do I compute and merge fields?

[jasonseminara]

I have the following schema, and I'd like to compute some fields:

type Place {
  placeID: Int!
  placeName: String!
}

type Sales {
  totalCommissionYTD: Float
}

type Seller {
    commission: Float
    placeID: Int
    
    # This field should be invented based on (commission / totalCommissionYTD)
    #  commissionPercentOfTotal: Float
   
    # This field should join Seller.placeID -> Place.ID
    # place: Place

} 

What how would I write this in yml?