create a solver which can efficiently add and remove elements instead of recomputing everything from scratch . concode_field_sep PlaceHolder placeHolder concode_field_sep LinearSolver<DenseMatrix64F> leastSquares concode_elem_sep LinearSolver<DenseMatrix64F> general concode_elem_sep LinearSolver<DenseMatrix64F> pseudoInverse concode_elem_sep LinearSolver<DenseMatrix64F> linear concode_elem_sep LinearSolver<DenseMatrix64F> symmPosDef concode_elem_sep LinearSolver<DenseMatrix64F> leastSquaresQrPivot