mlt_field field = mlt_tractor_field( MLT_TRACTOR( parent ) );
if ( mlt_properties_get_int( properties, "a_track" ) == mlt_properties_get_int( properties, "b_track" ) )
mlt_properties_set_int( properties, "b_track", mlt_properties_get_int( properties, "a_track" ) + 1 );
mlt_field field = mlt_tractor_field( MLT_TRACTOR( parent ) );
if ( mlt_properties_get_int( properties, "a_track" ) == mlt_properties_get_int( properties, "b_track" ) )
mlt_properties_set_int( properties, "b_track", mlt_properties_get_int( properties, "a_track" ) + 1 );