This data structure is difficult to understand, because you are describe what it does in terms of your application, but not in terms of the data structure.

Let me try to understand what it does:
- First it's growable, like a Cpp vector.
- Second it is sparse.
- Last it thresholds the data it gets in.

I don't think that anything more is required to understand this object. Am I wrong? If so we can try to rephrase the variable names and the docstring in these terms.

I wonder if the threshold should not