code: def _EOM(y,t,pot):
      
      l2= (y[0]**2.*y[3])**2.
      return [y[1],
              l2/y[0]**3.+_evaluateplanarRforces(pot,y[0],phi=y[2],t=t),
              y[3],
              1./y[0]**2.*(_evaluateplanarphiforces(pot,y[0],phi=y[2],t=t)-
                           2.*y[0]*y[1]*y[3])]
docs: NAME:
         _EOM
      PURPOSE:
         implements the EOM, i.e., the right-hand side of the differential 
         equation
      INPUT:
         y - current phase-space position
         t - current time
         pot - (list of) Potential instance(s)
         l2 - angular momentum squared
      OUTPUT:
         dy/dt
      HISTORY:
         2010-07-20 - Written - Bovy (NYU)