Coordinate values have to be unique and monotonic, because they define the order of the elements along an axis. Strings don't have a unique way of being ordered monotonically. Of course we could define which order CF was to use, but that's an extra complexity, and it would probably go wrong sometimes, so there has to be a good reason for introducing and depending on it. The usual rule in CF is that we don't add something unless there is a clear use-case for it. `string x(x)` and `char x(x)` are currently not allowed as a coordinate variables or auxiliary coordinate variables. Is there a circumstance where they are needed? No-one has asked for them up to now. I continue to think that we should disallow them as coordinate variables, because of the problem with monotonicity, and as auxiliary coordinate variables because they look like coordinate variables.
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