The last of these enumerations of grounds for jurisdiction of the federal courts includes several possible classes of cases. It is difficult to conceive of a controversy between a state and a foreign government. But if such controversy could arise, determinable by the courts, it would be within the possible jurisdiction of the federal courts. If a foreign government sought to sue the citizens of a state, such suit would be within the scope of federal jurisdiction. Controversies between a citizen of a state and a citizen or subject of a foreign government, that is, suits between citizens and aliens, are of frequent occurrence, and the propriety of placing them within the general scope of federal jurisdiction is self-evident.