Sovereignty — In matters of domestic jurisdiction — Procedural problems of tax assessment — Whether within competence of tax authorities of Federal Republic of Germany.

Treaties — Interpretation of — Principles and rules of — Strict interpretation of provision affecting taxing powers of sovereign State.

Arbitration — In general — Law applied by arbitral tribunals — Decisions of municipal courts as “ helpful guides ” in matters of substance.

Arbitration — Procedure — Competence — Arbitral Commission on Property, Rights and Interests in Germany — Interpretation of provisions on competence — Powers specifically granted to Commission — Whether grant is exclusive — Decisions of municipal courts — Whether relevant in defining jurisdiction of Commission.