Does Article 15(1) of the Convention to prevent double taxation between Belgium and Luxembourg 1970, interpreted as allowing a restriction on the power of taxation of the source State in respect of the remuneration of an employee residing in Belgium and performing his activity for a Luxembourgish employer in proportion to the activity performed on Luxembourgish territory, interpreted as allowing a power of taxation to be attributed to the State of residence in respect of the amount of remuneration relating to the activity performed outside Luxembourgish territory, interpreted as requiring the employee to be permanently and every day physically present at the seat of his employer when it is not disputed that he regularly travels there as the result of a judicial assessment, conducted with flexibility, on the basis of objective and verifiable evidence and interpreted as requiring courts and tribunals to evaluate the existence and relevance of the services provided there and elsewhere, day by day, with a view to establishing a proportion over 220 business days, infringe Article 45 TFEU in that it constitutes a tax hindrance which discourages activities across borders and breaches the general principle of legal certainty in that it does not provide for a stable and secure regime of exemption of the entirety of the remuneration received by a Belgian resident employed by an employer whose actual seat is situated in Luxembourg and places him at risk of double taxation of all or part of his income and subjects him to an unpredictable legally uncertain regime?