anchor.diff:      real, intent(in) :: x
       real, intent(in) :: h
       integer :: n, m
  -    real, intent(in) :: y(0:n-1,0:m-1)
  +!    real, intent(in) :: y(0:n-1,0:m-1)
   
   
       real :: hh, h6, xh
  
positive.code: subroutine rk4stepks(x, y, h, n, m, yout, derivs)
  !   Do one step of the classical 4th order Runge Kutta method,
  !   starting from y at x with time step h and derivatives given by derivs
      implicit none
  
negative.code: 
      !Accumulate increments with proper weights
      !yout = y + h6*(dydx + dyt + 2*dym)
      yout = x
  
      return
      end subroutine
  