AbstractSyntaxNotation1Element.external

Decodes an $(MONO EXTERNAL), which is a constructed data type, defined in the https://www.itu.int, International Telecommunications Union's https://www.itu.int/rec/T-REC-X.680/en, X.680.

The specification defines $(MONO EXTERNAL) as:

EXTERNAL := [UNIVERSAL 8] IMPLICIT SEQUENCE {
                identification CHOICE {
                    syntax OBJECT IDENTIFIER,
                    presentation-context-id INTEGER,
                    context-negotiation SEQUENCE {
                        presentation-context-id INTEGER,
                        transfer-syntax OBJECT IDENTIFIER } },
                data-value-descriptor ObjectDescriptor OPTIONAL,
                data-value OCTET STRING }

This assumes AUTOMATIC TAGS, so all of the identification choices will be context-specific and numbered from 0 to 2.