These are the steps to take when another semantic data type must be added to
an existing set:
    itemization(
    it() Add a tag name representing the new semantic value type to the
tt(enum class Tag) (section ref(TAG));
    it() Add a specialization to the tt(TagTrait) trait class (section
ref(TAGTRAIT)) defining the added semantic value's data type, and whether or
not the data on the parser's semantic stack should be considered (im)mutable.
    )
