Combines two geometries which each other. 
The free function union calculates the spatial set theoretic union of two geometries. 
- Template Parameters
- 
  
    | Geometry1 | Any type fulfilling a Geometry Concept |  | Geometry2 | Any type fulfilling a Geometry Concept |  | Collection | output collection, either a multi-geometry, or a std::vector<Geometry> / std::deque<Geometry> etc |  
 
- Parameters
- 
  
    | geometry1 | A model of the specified concept |  | geometry2 | A model of the specified concept |  | output_collection | the output collection |  
 
- Note
- Called union_ because union is a reserved word.
- Examples: 
- 03_polygon_example.cpp.