Answer the question from the given passage. Your answer should be directly extracted from the passage, and it should be a single entity, name, or number, not a sentence.
Q: Passage: Since its founding, the EU has operated among an increasing plurality of national and globalising legal systems. This has meant both the European Court of Justice and the highest national courts have had to develop principles to resolve conflicts of laws between different systems. Within the EU itself, the Court of Justice's view is that if EU law conflicts with a provision of national law, then EU law has primacy. In the first major case in 1964, Costa v ENEL, a Milanese lawyer, and former shareholder of an energy company, named Mr Costa refused to pay his electricity bill to Enel, as a protest against the nationalisation of the Italian energy corporations. He claimed the Italian nationalisation law conflicted with the Treaty of Rome, and requested a reference be made to both the Italian Constitutional Court and the Court of Justice under TFEU article 267. The Italian Constitutional Court gave an opinion that because the nationalisation law was from 1962, and the treaty was in force from 1958, Costa had no claim. By contrast, the Court of Justice held that ultimately the Treaty of Rome in no way prevented energy nationalisation, and in any case under the Treaty provisions only the Commission could have brought a claim, not Mr Costa. However, in principle, Mr Costa was entitled to plead that the Treaty conflicted with national law, and the court would have a duty to consider his claim to make a reference if there would be no appeal against its decision. The Court of Justice, repeating its view in Van Gend en Loos, said member states 'albeit within limited spheres, have restricted their sovereign rights and created a body of law applicable both to their nationals and to themselves' on the 'basis of reciprocity'. EU law would not 'be overridden by domestic legal provisions, however framed... without the legal basis of the community itself being called into question.' This meant any 'subsequent unilateral act' of the member state inapplicable. Similarly, in Amministrazione delle Finanze v Simmenthal SpA, a company, Simmenthal SpA, claimed that a public health inspection fee under an Italian law of 1970 for importing beef from France to Italy was contrary to two Regulations from 1964 and 1968. In 'accordance with the principle of the precedence of Community law,' said the Court of Justice, the 'directly applicable measures of the institutions' (such as the Regulations in the case) 'render automatically inapplicable any conflicting provision of current national law'. This was necessary to prevent a 'corresponding denial' of Treaty 'obligations undertaken unconditionally and irrevocably by member states', that could 'imperil the very foundations of the' EU. But despite the views of the Court of Justice, the national courts of member states have not accepted the same analysis. Question: Which entities have had to develop principles dedicated to conflict resolution between laws of different systems?
A:
the European Court of Justice and the highest national courts