PROGRAM wheel
      IMPLICIT NONE
      REAL*8 dt, omega, r, t, vcm, x, xmax, y
      INTEGER IR_
      DATA x/0.0D0/
      CALL GWopen(IR_, 0)
      CALL initial(r,omega,vcm,t,dt,xmax)
      CALL GWsetpen(IR_, -1, -1, -1, -1)
      DO WHILE(x .LE. xmax)
         x = -r*cos(omega*t) + vcm*t
         y = r*sin(omega*t)
*        note the semicolon after PLOT x,y
         CALL GWline2(IR_, REAL(x), REAL(y)) ! abbreviation of PLOT POINT:
         t = t + dt
      END DO
      CALL GWquit(IR_)
      END
C
      SUBROUTINE initial(r,omega,vcm,t,dt,xmax)
      IMPLICIT NONE
      REAL*8 r, omega, vcm, t, dt, xmax
      REAL mx, xmin
      INTEGER IR_
      t = 0
      r = 1                                 ! radius of wheel
      omega = 1                             ! angular frequency of wheel
      vcm = r*omega                         ! velocity of center of mass 
      dt = 0.01D0                           ! time increment
      xmin = -1
      xmax = 50
      mx = 0.01D0*(xmax - xmin)
      CALL GWindow(IR_, xmin-mx, -4.0, REAL(xmax+mx), 4.0)
*     draw ground at y = -r (bottom of wheel)
      CALL GWline(IR_, xmin, REAL(-r), REAL(xmax), REAL(-r))
      END