It was declared in the .adb, thus not visible when the module was
imported from an Ada file

must be set to true only after elaboration (complete execution of the
first transition).

Requires asn1scc 3.3.04 or more recent

TypeA ::= SEQUENCE {
  a SEQUENCE (SIZE (2)) OF BOOLEAN
}

TypeB ::= SEQUENCE {
  a SEQUENCE (SIZE(2)) OF BOOLEAN
}

dcl a TypeA, b TypeB;
a!a := b!a
now rightfully raises an error, even if the types seem semantically
equivalent- they are two different types, and Ada would not compile
accept that.

mychoice!a!b := 5was not properly spotted as an error in
selector_expression. the right syntax is mychoice := a: { b 5 }  (ASN.1
Value notation)