def pOparapar(self,Opar,apar):
        """
        NAME:

           pOparapar

        PURPOSE:

           return the probability of a given parallel (frequency,angle) offset pair

        INPUT:

           Opar - parallel frequency offset (array) (can be Quantity)

           apar - parallel angle offset along the stream (scalar) (can be Quantity)

        OUTPUT:

           p(Opar,apar)

        HISTORY:

           2015-11-17 - Written - Bovy (UofT)

        """
        if _APY_LOADED and isinstance(Opar,units.Quantity):
            Opar= Opar.to(1/units.Gyr).value\
                /bovy_conversion.freq_in_Gyr(self._vo,self._ro)
        if _APY_LOADED and isinstance(apar,units.Quantity):
            apar= apar.to(units.rad).value
        if isinstance(Opar,(int,float,numpy.float32,numpy.float64)):
            Opar= numpy.array([Opar])
        out= numpy.zeros(len(Opar))
        # Compute ts and where they were at impact for all
        ts= apar/Opar
        apar_impact= apar-Opar*self._timpact
        dOpar_impact= self._kick_interpdOpar(apar_impact)
        Opar_b4impact= Opar-dOpar_impact
        # Evaluate the smooth model in the two regimes:
        # stripped before or after impact
        afterIndx= (ts < self._timpact)*(ts >= 0.)
        out[afterIndx]=\
            super(streamgapdf,self).pOparapar(Opar[afterIndx],
                                              apar)
        out[True^afterIndx]=\
            super(streamgapdf,self).pOparapar(Opar_b4impact[True^afterIndx],
                                              apar_impact[True^afterIndx],
                                              tdisrupt=
                                              self._tdisrupt-self._timpact)
        return out
NAME:

           pOparapar

        PURPOSE:

           return the probability of a given parallel (frequency,angle) offset pair

        INPUT:

           Opar - parallel frequency offset (array) (can be Quantity)

           apar - parallel angle offset along the stream (scalar) (can be Quantity)

        OUTPUT:

           p(Opar,apar)

        HISTORY:

           2015-11-17 - Written - Bovy (UofT)