def determine_xsi_type_override(field_data, field_def)
      result = nil
      if field_data.kind_of?(Hash) and field_data.include?(:xsi_type)
        result = field_data[:xsi_type]
      end
      return result
    end
def determine_xsi_type_override(field_data, field_def)
      result = nil
      if field_data.kind_of?(Hash) and field_data.include?(:xsi_type)
        result = field_data[:xsi_type]
      end
      return result
    end
Determines an xsi:type override for for the field. Returns nil if no
 override found.