/** Returns the first value from a key. The key must exist and be
non-empty. */
const std::string &get_value(const Target *, const std::string &);
/// Returns the values from a key. The key must exist.
/** Returns the first value from a key. The key must exist and be
non-empty. */
const std::string &get_value(const Target *, const std::string &);
/// Returns the values from a key. The key must exist.