code: ModuleMarshal marshal(Class<?> sourceType,
                          Class<?> targetType,
                          Class<?> declaredTargetType)
    {
      ImportKey key = new ImportKey(sourceType, targetType);
      
      ModuleMarshal marshal = _marshalMap.get(key);
      
      if (marshal != null) {
        return marshal;
      }
      
      marshal = marshalImpl(sourceType, targetType, declaredTargetType);
      
      _marshalMap.putIfAbsent(key, marshal);
      
      return marshal;
    }
docs: Returns the marshal to convert from the sourceType to the targetType.