program prog082
      .....
 10   continue
         call onestep(x,y,vx,vy,t,dt,ga)
         .....
      if( y .ge. 0.0d0 ) goto 10
c
      call solvey0(x,y,vx,vy,t,ga)
      xf=x
      write(*,100) theta, xf, t
      ....
      end
c **********************************************************************
      subroutine solvey0(x,y,vx,vy,t,ga)
c **********************************************************************
      .....
c
      eps = 1.0d-10
      do n = 1, 1000
        if(abs(y).lt.eps) goto 20
        dt = .....
        call onestep(x,y,vx,vy,t,dt,ga) ! 時間を dt だけ進ませる
      enddo
        write(*,*) " I cannot find the position where y = 0."
20    continue
c
      end