SACM24 — ExpressionLangString should not exist

There is no reason to have both LangString and ExpressionLangString, given the constraint on ExpressionLangString that the expression and the content cannot both have values. If you insist on having both, have a common base class that includes only the 'lang' attribute - then LangString can have the content field and ExpressionLangString can have the 'expression' field.