code: func (m *Mapper) TypeMap(t reflect.Type) *StructMap {
  	m.mutex.Lock()
  	mapping, ok := m.cache[t]
  	if !ok {
  		mapping = getMapping(t, m.tagName, m.mapFunc, m.tagMapFunc)
  		m.cache[t] = mapping
  	}
  	m.mutex.Unlock()
  	return mapping
  }
docs: // TypeMap returns a mapping of field strings to int slices representing
  // the traversal down the struct to reach the field.