The type of external references shouldn't be "open"

[pvdrz]

According to the spec for 1.3, 1.4 and 1.5. The type field of an external reference must be one of many fixed strings. It is mentioned that if none of the types is adequate "other" should be used. However, the current implementation allows to use any string as a valid type.

This could be a misinterpretation from my part so I'd like to know your thoughts on the matter.