When there is no clear canonical way of implementing something, adding it to a programming language (or a standard library) is risky. All too often, you realize too late that you made a wrong choice, and then you add a second version. And a third. And so on. And then you end up with a confusing language full of newbie traps.

Graphs are a good example, as they are a large family of related structures. For example, are the edges undirected, directed, or something more exotic? Do the nodes/edges have identifiers and/or labels? Are all nodes/edges of the same type, or are there multiple types? Can you have duplicate edges between the same nodes? Does that depend on the types of the nodes/edges, or on the labels?