arguments: m:SparseMutableMatrix c1:int c2:int >> r:int c:int
Find a unit (currently: only checks for 1, -1), m[r,c], which would minimize 'fill-ins', at least in the range of columns c1..c2. If no 1 or -1 is found in the matrix in this range of columns, then r and c are both set to -1.