my new adt, what should i name it

[Skaperen]

i'm making a new abstract data type. it might exist, already. it is a list that expands as items are stored to it. if the location already exists, it replaces what is there. it not, it expands the list (virtually). you can get what's there at any time. it keeps data in a sparse way, allowing the caller to see extents of data and holes, including the highest and lowest extents. can you tell me what to name this or what the existing name is?