Here are two different ways we could leverage our triangle! function to make a big colored triangle, thought of as three colored triangles which a related by a kind of gluing relationship.

The first one requires us to modify triangle!. No longer does it just add a triangle to some preexisting database instance, but it allows one to pass in parameters so that the triangle has the option of reusing existing vertices. Not only did we have to substantial refactor of the function, it was also convenient to change the behavior of the old function a bit (by having it return vertices rather than the graph itself), so there may be other places in our codebase where we have to fix broken code.

The second one doesn’t require us to modify the function: it just creates three disjoint triangles, and then ‘manually’ modifies the result to manipulate the foriegn keys and finally delete the isolated vertices. Although we did not need to modify the function, we certainly needed a deep understanding of its implementation in order to do this, plus this code is very clunky and not at all re-usable for other problems (whereas the first solution has potential to be used for other compositions of triangles).