action fwdBase ()
{
    static const var angForce = 600;
    
    while (1)
    {
        // rotate base with A and D
        var f = key_a - key_d;
        if (f != 0)
        {
            // open constraint, if closed
            if (!isFwdBaseOpen(my))
                openFwdBase(my);
            
            // rotate base
            var vt = f * angForce * time_step;
            pXent_setangvelocity(my, vector(0, 0, vt));
        }
        else // close constraint, when no force is applied
            if (isFwdBaseOpen(my))
                closeFwdBase(my);
    
        wait(1);
    }
}
    
    void fwdBaseMakeContraint (ENTITY* eBase, var limit)
    {
        // use cage upward vector is hinge direction
        
        VECTOR vAxis;
        vec_set(&vAxis, vector(0,0,-1));
        vec_rotate(&vAxis, eBase->parent->pan);

        // create stiff hinge constraint for the base
        pXcon_add(PH_HINGE, eBase, eBase->parent, 0); 
        pXcon_setparams1(eBase, eBase->x, &vAxis, NULL);
        pXcon_setparams2(eBase, vector(-limit, limit, 100), NULL, NULL);
    }

    // returns if base constraint is open
    BOOL isFwdBaseOpen (ENTITY* base)
    {
        static VECTOR vCon;
        pXcon_getposition(base, &vCon);
        
        BOOL isOpen = false;
        
        if ((vCon.x == -180) && (vCon.y == 180))
            isOpen = true;
            
        return(isOpen);
    }
    
    // opens base constraint
    void openFwdBase (ENTITY* base)
    {
        pXcon_remove(base);
        fwdBaseMakeContraint(base, 180);
    }
    
    // closes base constraint
    void closeFwdBase (ENTITY* base)
    {
        pXcon_remove(base);
        fwdBaseMakeContraint(base, 5);
    }