c_get_tridiag_block Subroutine

subroutine c_get_tridiag_block(Nblock,Nsize,Amat,sub,diag,over)
  integer                                          :: Nblock
  integer                                          :: Nsize
  complex(8),dimension(Nblock*Nsize,Nblock*Nsize)     :: Amat
  complex(8),dimension(Nblock-1,Nsize,Nsize)          :: sub
  complex(8),dimension(Nblock,Nsize,Nsize)            :: diag
  complex(8),dimension(Nblock-1,Nsize,Nsize),optional :: over
  complex(8),dimension(Nblock-1,Nsize,Nsize)          :: over_
  integer                                          :: i,j,iblock,is,js
  do iblock=1,Nblock-1
     do i=1,Nsize
        do j=1,Nsize
           is = i + (iblock-1)*Nsize
           js = j + (iblock-1)*Nsize
           Sub(iblock,i,j)   = Amat(Nsize+is,js)
           Diag(iblock,i,j)  = Amat(is,js)
           Over_(iblock,i,j) = Amat(is,Nsize+js)
        enddo
     enddo
  enddo
  do i=1,Nsize
     do j=1,Nsize
        is = i + (Nblock-1)*Nsize
        js = j + (Nblock-1)*Nsize
        Diag(Nblock,i,j) = Amat(is,js)
     enddo
  enddo
  if(present(over))over=over_
end subroutine c_get_tridiag_block