int FirstBit() const
The FirstBit method returns the index of the first bit, i.e.
the set bit with the lowest index, in the OEBitVector. Bit position
indices are numbered from zero, and have a maximum of
OEBitVector::GetSize()-1.
If the OEBitVector has no bits set, i.e.
OEBitVector::IsEmpty returns true, this method return will return
the value -1.