/** @param periods Set of periods in ascending order of from time */
template <class T>
std::list<TimePeriod<T> > coalesce (std::list<TimePeriod<T> > periods)
/** @param periods Set of periods in ascending order of from time */
template <class T>
std::list<TimePeriod<T> > coalesce (std::list<TimePeriod<T> > periods)