class ViewModel::References

it by calling the given block.
under which the data is stored. If the data is not present, will compute
Takes a reference to a thing that is to be shared, and returns the id

def add_reference(value)
  ref = @ref_by_value[value]
  unless ref.present?
    ref = new_ref!(value)
    @ref_by_value[value] = ref
    @value_by_ref[ref] = value
  end
  ref
end