Types must be identifiers
The name of the type must be an identifier. That is, it must follow certain rules.
Definition of identifier
An identifier, in Prolog+CG, is any sequence of:
The first two characters must be letters.
For example, the following are all identifiers:
These are all identifiers because they conform to the rule given above.
However, the following are not identifiers:
Thus an identifier is a sequence of letters, digits, and underscores, where the first two characters are letters.
Next, we give more hints and further rules for constructing type-hierarchies.
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