Eigen
3.3.4

Generic expression of a partially reduxed matrix.
MatrixType  the type of the matrix we are applying the redux operation 
MemberOp  type of the member functor 
Direction  indicates the direction of the redux (Vertical or Horizontal) 
This class represents an expression of a partial redux operator of a matrix. It is the return type of some VectorwiseOp functions, and most of the time this is the only way it is used.
Inherits type< PartialReduxExpr< MatrixType, MemberOp, Direction > >, and Eigen::internal::no_assignment_operator.