International organization — European Coal and Steel Community — Decisions and opinions of High Authority — Opinion of High Authority on investment programme of enterprise submitted under Article 54 (4) of Treaty — Whether such opinion can be subject of appeal for annulment — Meaning of “opinion” within Article 54 (4) — Whether opinion can constitute disguised decision — Criteria for classifying act of High Authority as a decision — Time-limits for taking action — Whether there is a limit for issuance of opinion under Article 54 (4) — Relevance of principles of sound administration — Effect of non-observance of time-limit on legal nature of act — Effect of absence of reasons for act — Reasons as constitutive element of act — Effect of finding by Court that appeal for annulment inadmissible because brought against an opinion and not against a decision — Whether opinion can have legally binding effects after such finding.