Changing m_counters.begin() to begin(m_counters) fixes a very weird range-v3 compilation problem observed in LHCb gcc8(.2) debug builds.
m_counters.begin()
begin(m_counters)
I also replace accumulate with count_if, because, to quote @graven, all interesting loops have already been written.
accumulate
count_if
CC: @graven, @cattanem