What is and is not a series
The series! typeset is used to define the argument types for a variety of functions.
Currently, the definition of series! itself comes from deep within the system. It includes various datatypes that use the internal series storage mechanism. However, those datatypes may use different sets of access functions. For example, the bitset! datatype. It does not make sense to refer to the head and tail (or other series-index related functions) of a bitset because they are irrelevant to its operation. So, is a bitset a series or not?
Fortunately, we can resolve this issue because we have precise control over the definition of the series! typeset and the series? type check. The suggestion has been made to remove types like bitset!, image!, and a few others from the series! definition.
That suggestion makes sense, but I wanted to gather your comments on it before proceeding.
Keep in mind that in R3 we can define any typeset we need. So, if we want (for some reason) a special typeset, such as series-based!, we can define that separately, and it requires very little overhead.