ElementType Enumeration

(Is a logical combination of the ElementTypeDescriptor field which defines the major type and the size field in the binary format; and the size field being made up of the SizeIndex field and any additional length bytes.

Note, the values here are not the numerical bitwise combination of the ElementTypeDescriptor and SizeIndex fields as they appear in the encoded protocol. It was simpler to assign arbitrary values here as firstly we wanted zero to be the 'Unknown' value, which conflicts with Nil's bitwise value; but also because the TextString, sequence and Url types can have various SizeIndex values and thus they wouldn’t be easily representable by one value here).

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