module lsqr {
  logical, parameter, private :: T = .true., F = .false.
  private                     :: normalize
contains
    subroutine solver( oper, x, dat, niter)  {
       interface {
           integer function oper( adj, add, x, dat)  {
           logical, intent( in) :: adj, add
           real, dimension (:)  :: x, dat
                }
           }
    real, dimension (:), intent (in)    :: dat
    real, dimension (:), intent (out)   :: x
    integer, intent (in)                :: niter
    real, dimension ( size( x  ))       :: w, v
    real, dimension ( size( dat))       :: u
    integer                             :: iter, stat
    double precision                    :: alfa, beta, rhobar, phibar
    double precision                    :: c, s, teta, rho, phi, t1, t2
    u = dat;   x = 0.    ;   call normalize( u, beta)
    stat = oper( T,F,v,u);   call normalize( v, alfa)
    w = v
    rhobar = alfa
    phibar = beta
    do iter = 1, niter {
        u = - alfa * u;  stat = oper( F, T, v, u);  call normalize( u, beta)
        v = - beta * v;  stat = oper( T, T, v, u);  call normalize( v, alfa)
        rho = sqrt( rhobar*rhobar + beta*beta)
        c = rhobar/rho;   s = beta/rho;   teta = s * alfa
        rhobar = - c * alfa;   phi = c * phibar;   phibar = s * phibar
        t1 = phi/rho;  t2 = -teta/rho
        x = x + t1 * w
        w = v + t2 * w
        }
    }
  subroutine normalize( vector, size) {
    real, dimension (:), intent (inout) :: vector
    double precision,    intent (out)   :: size
    size = sqrt( sum( dprod( vector, vector)))
    vector = vector / size
    }
}