Principle of non-discrimination - Directive 2006/54/EC of the European Parliament and of the Council of 5 July 2006: “it is also appropriate to make express provision for the protection of the employment rights of women on maternity leave and in particular their right to return to the same or an equivalent post, to suffer no detriment in their terms and conditions as a result of taking such leave and to benefit from any improvement in working conditions to which they would have been entitled during their absence”