If you choose to deal with men by means of compulsion, do so.

The simplest of all, for a lone man without an army or any means of compulsion, is to ride up to the gate and ask to be let in.

Experience has taught that mere requests for such information often are unavailing, and also that information which is volunteered is not always accurate or complete; so some means of compulsion are essential to obtain what is needed.

Article 13 allows the service of documents and the taking of evidence by consular and diplomatic agents in the state in which they are accredited, provided their acts are not contrary to the law of that state and involve no means of compulsion.

In the landmark case McGrain v. Daugherty (1927), the Supreme Court firmly established a comprehensive reading of congressional investigative power, noting that a "legislative body cannot legislate in the absence of information," and that "some means of compulsion are essential to obtain what is needed."

The system is described by the means of constraints.

In many cases, such as in Constraint Satisfaction Problems, one can dramatically reduce the search space by means of Constraint propagation, that is efficiently implemented in Constraint programming languages.

The measures of compulsion will be those prescribed by law for use in internal proceedings.

The requested authority is required by Article 10 to apply the same measures of compulsion against an unwilling witness as it would do under its internal law in local proceedings.

The limitation to procedural acts 'of a merely formal nature' seems to be explained, albeit indirectly, by the provision in Article 3 that the convention is not to apply to acts involving measures of compulsion.

If the requested State has not filed a declaration under Article 18, and measures of compulsion are not available by other means under internal law, the request must state that he is not compelled to appear or to give evidence.