+== Paramaters ===
+<username> is a textual username unless the user does not have a username,
+in which case the username is of the form '#<id>' where <id> is a unique
+account number assigned by the server.
+<card_id_hex> is the hexadecimal byte stream representation of a MIFARE card.
+<item_id> is an item ID of the form '<type>:<id>', where <type> is an
+alpha-numeric diriver identifier, and <id> is an non-negative integer assigned
+by the driver.
+<ammount> is an integer number of cents representing the size of a transactio.
+Individual commands may restrict the range of the ammount to non-negative
+numbers.
+
+