subroutine get_excitation_general(key_i,key_j, Nint,degree_array,holes_array, particles_array,phase)
 use bitmasks
 BEGIN_DOC
! returns the array, for each spin, of holes/particles between key_i and key_j 
!
! with the following convention: a^+_{particle} a_{hole}|key_i> = |key_j>
 END_DOC
  include 'utils/constants.include.F'
 implicit none
 integer, intent(in)            :: Nint
 integer(bit_kind), intent(in)  :: key_j(Nint,2),key_i(Nint,2)
 integer, intent(out)           :: holes_array(100,2),particles_array(100,2),degree_array(2)
 double precision, intent(out)  :: phase
 integer :: ispin,k,i,pos
 integer(bit_kind) :: key_hole, key_particle
 integer(bit_kind) :: xorvec(N_int_max,2)
 holes_array = -1
 particles_array = -1
 degree_array = 0
  do i = 1, N_int
   xorvec(i,1) = xor( key_i(i,1), key_j(i,1))
   xorvec(i,2) = xor( key_i(i,2), key_j(i,2))
   degree_array(1) += popcnt(xorvec(i,1))
   degree_array(2) += popcnt(xorvec(i,2))
  enddo
  degree_array(1) = shiftr(degree_array(1),1)
  degree_array(2) = shiftr(degree_array(2),1)
  
 do ispin = 1, 2
  k = 1
  !!! GETTING THE HOLES 
  do i = 1, N_int
   key_hole = iand(xorvec(i,ispin),key_i(i,ispin))
   do while(key_hole .ne.0_bit_kind)
    pos = trailz(key_hole)
    holes_array(k,ispin) = 1+ bit_kind_size * (i-1) + pos
    key_hole = ibclr(key_hole,pos)
    k += 1
    if(k .gt.100)then
     print*,'WARNING in get_excitation_general'
     print*,'More than a 100-th excitation for spin ',ispin
     print*,'stoping ...'
     stop
    endif
   enddo 
  enddo
 enddo
 do ispin = 1, 2
  k = 1
  !!! GETTING THE PARTICLES
  do i = 1, N_int
   key_particle = iand(xor(key_i(i,ispin),key_j(i,ispin)),key_j(i,ispin))
   do while(key_particle .ne.0_bit_kind)
    pos = trailz(key_particle)
    particles_array(k,ispin) = 1+ bit_kind_size * (i-1) + pos
    key_particle = ibclr(key_particle,pos)
    k += 1
    if(k .gt.100)then
     print*,'WARNING in get_excitation_general '
     print*,'More than a 100-th excitation for spin ',ispin
     print*,'stoping ...'
     stop
    endif
   enddo
  enddo 
 enddo
 integer :: h,p, i_ok
 integer(bit_kind), allocatable :: det_i(:,:),det_ip(:,:)
 integer                        :: exc(0:2,2,2)
 double precision :: phase_tmp
 allocate(det_i(Nint,2),det_ip(N_int,2))
 det_i = key_i
 phase = 1.d0
 do ispin = 1, 2
  do i = 1, degree_array(ispin)
   h = holes_array(i,ispin)
   p = particles_array(i,ispin)
   det_ip = det_i
   call do_single_excitation(det_ip,h,p,ispin,i_ok)
   if(i_ok == -1)then
     print*,'excitation was not possible '
     stop
   endif
   call get_single_excitation(det_i,det_ip,exc,phase_tmp,Nint)
   phase *= phase_tmp
   det_i = det_ip
  enddo
 enddo

end

subroutine get_holes_general(key_i, key_j,Nint, holes_array)
 use bitmasks
 BEGIN_DOC
! returns the array, per spin, of holes between key_i and key_j 
!
! with the following convention: a_{hole}|key_i> --> |key_j>
 END_DOC
 implicit none
 integer, intent(in)            :: Nint
 integer(bit_kind), intent(in)  :: key_j(Nint,2),key_i(Nint,2)
 integer, intent(out)           :: holes_array(100,2)
 integer(bit_kind) :: key_hole
 integer :: ispin,k,i,pos
 holes_array = -1
 do ispin = 1, 2
  k = 1
  do i = 1, N_int
   key_hole = iand(xor(key_i(i,ispin),key_j(i,ispin)),key_i(i,ispin))
   do while(key_hole .ne.0_bit_kind)
    pos = trailz(key_hole)
    holes_array(k,ispin) = 1+ bit_kind_size * (i-1) + pos
    key_hole = ibclr(key_hole,pos)
    k += 1
    if(k .gt.100)then
     print*,'WARNING in get_holes_general'
     print*,'More than a 100-th excitation for spin ',ispin
     print*,'stoping ...'
     stop
    endif
   enddo 
  enddo
 enddo
end

subroutine get_particles_general(key_i, key_j,Nint,particles_array)
 use bitmasks
 BEGIN_DOC
! returns the array, per spin, of particles between key_i and key_j 
!
! with the following convention: a^dagger_{particle}|key_i> --> |key_j>
 END_DOC
 implicit none
 integer, intent(in)            :: Nint
 integer(bit_kind), intent(in)  :: key_j(Nint,2),key_i(Nint,2)
 integer, intent(out)           :: particles_array(100,2)
 integer(bit_kind) :: key_particle
 integer :: ispin,k,i,pos
 particles_array = -1
 do ispin = 1, 2
  k = 1
  do i = 1, N_int
   key_particle = iand(xor(key_i(i,ispin),key_j(i,ispin)),key_j(i,ispin))
   do while(key_particle .ne.0_bit_kind)
    pos = trailz(key_particle)
    particles_array(k,ispin) = 1+ bit_kind_size * (i-1) + pos
    key_particle = ibclr(key_particle,pos)
    k += 1
    if(k .gt.100)then
     print*,'WARNING in get_holes_general'
     print*,'More than a 100-th excitation for spin ',ispin
     print*,'Those are the two determinants'
     call debug_det(key_i, N_int)
     call debug_det(key_j, N_int)
     print*,'stoping ...'
     stop
    endif
   enddo 
  enddo
 enddo
end

subroutine get_phase_general(key_i,Nint,degree, holes_array, particles_array,phase)
 implicit none
 integer, intent(in)            :: degree(2), Nint
 integer(bit_kind), intent(in)  :: key_i(Nint,2)
 integer, intent(in)            :: holes_array(100,2),particles_array(100,2)
 double precision, intent(out)  :: phase
 integer :: i,ispin,h,p, i_ok
 integer(bit_kind), allocatable :: det_i(:,:),det_ip(:,:)
 integer                        :: exc(0:2,2,2)
 double precision :: phase_tmp
 allocate(det_i(Nint,2),det_ip(N_int,2))
 det_i = key_i
 phase = 1.d0
 do ispin = 1, 2
  do i = 1, degree(ispin)
   h = holes_array(i,ispin)
   p = particles_array(i,ispin)
   det_ip = det_i
   call do_single_excitation(det_ip,h,p,ispin,i_ok)
   if(i_ok == -1)then
     print*,'excitation was not possible '
     stop
   endif
   call get_single_excitation(det_i,det_ip,exc,phase_tmp,Nint)
   phase *= phase_tmp
   det_i = det_ip
  enddo
 enddo

end