code: func (m *modelPrinter) VisitMap(om *proto.Map) {
  	m.Descriptions = append(m.Descriptions, om.GetDescription())
  	if m.Type == "" {
  		m.Type = GetTypeName(om)
  	}
  	om.SubType.Accept(m)
  }
docs: // VisitMap recurses inside the subtype, while collecting the type if
  // not done yet, and the description.