c_build_tridiag_block Function

function c_build_tridiag_block(Nblock,Nsize,sub,diag,over) result(Amat)
  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
  over_=sub;if(present(over))over_=over
  !
  Amat=0d0
  !
  do iblock=1,Nblock-1
     do i=1,Nsize
        do j=1,Nsize
           is = i + (iblock-1)*Nsize
           js = j + (iblock-1)*Nsize
           Amat(Nsize+is,js) = Sub(iblock,i,j)
           Amat(is,js)       = Diag(iblock,i,j)
           Amat(is,Nsize+js) = Over_(iblock,i,j)
        enddo
     enddo
  enddo
  do i=1,Nsize
     do j=1,Nsize
        is = i + (Nblock-1)*Nsize
        js = j + (Nblock-1)*Nsize
        Amat(is,js)       = Diag(Nblock,i,j)
     enddo
  enddo
end function c_build_tridiag_block