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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section VIII : Armistice -- Capitulation - Art. 141.
Art. 141. It is incumbent upon the contracting parties of an armistice to stipulate what intercourse of persons or traffic between the inhabitants of the territories occupied by the hostile armies shall be allowed, if any.
If nothing is stipulated the intercourse remains suspended, as during actual hostilities.