Fix to issue #476.

backup/moodle2/backup_qtype_stack_plugin.class.php

backup/moodle2/restore_qtype_stack_plugin.class.php

db/install.xml

db/upgrade.php

doc/en/Authoring/Options.md

doc/en/Developer/Development_track.md

edit_stack_form.php

lang/en/qtype_stack.php

questiontype.php

settings.php

stack/maxima/stackmaxima.mac

stack/options.class.php

tests/cassession2_test.php

tests/helper.php

tests/questiontype_test.php

version.php