protected DataType leastGeneralOf(DataType left, DataType right) {
        if (left == null || right == null) return null;
        if (left.isAssignableTo(right)) return left;
        if (right.isAssignableTo(left)) return right;
        throw new VerificationException(this, left.getName() + " is incompatible with " + right.getName());
    }
Creates an expression with simple lingustics for testing
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/Expression.java