Potential test cases

Explicit key in flow sequence

libyaml had problems with it

[?] # valid

[?]] # invalid

% echo '[?]' | docker run -i --rm yamlio/alpine-runtime-perl /yaml/bin/perl-refparser-event
Parser finished before end of input at /perl5/lib/perl5/YAML/Parser.pm line 4056, <> line 1.

% echo '[?]' | docker run -i --rm yamlio/alpine-runtime-static /yaml/bin/c-libfyaml-event
+STR
+DOC
+SEQ []
=VAL :?
-SEQ
-DOC
-STR