TVecInt BitMask { get; set; }
Bit vector holding 1 for included and 0 for excluded vars.
Result: stored in self (calling object), returns self for chaining
Remarks:
Initialize the BitMask with BitMask.BitCount and then set BitMask.Bits[i] and set AutoInitBitMask property to false. When doing forward search, those variables already 1 will not be excluded. When doing backward search, the variables already 0 will not be included.