Treaties — Special kinds of — Double taxation conventions — Income-tax Convention between United States and Switzerland — Definition of term “resident” and of clause “services performed … under contract with a … corporation … of Switzerland” — Application of exemption provision to Swedish national earning income in United States but alleging Swiss residence and contract with Swiss corporation — Interpretation in view of purposes of income-tax conventions — Relevance of determination of taxpayer's residence by Swiss authorities — The law of the United States of America.