protected DataType leastGeneralNonNullOf(DataType left, DataType right) {
        if (left == null) return right;
        if (right == null) return left;
        return leastGeneralOf(left, right);
    }
Creates an expression with simple lingustics for testing
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/Expression.java