A stringed musical instrument employs string tensioners to support and apply tension to corresponding musical strings. First selected rows from the first plurality of rows that meet a query predicate are stored into a result set. An expressive policy-specification system is separated from the underlying information-flow tracking mechanism. In this manner, the user’s privacy is protected by preventing the user’s identity, routes, and activities from being correlated with hisher device’s address. A second filter averages a plurality of cardiac cycles and cardiac parameters are extracted from the averaged cardiac signal. The multi-layer core consists of a rubber center and a rubber outer core layer that are both soft relative to a hard, thermoplastic intermediate core layer.