switch(FEAT_, SELF, generation4) ...
switch(FEAT_, SELF, generation3) ...
switch(FEAT_, SELF, generation2) ...
switch(FEAT_, SELF, generation1) ...

#define YEAR_GEN2 1930
#define YEAR_GEN3 1960
#define YEAR_GEN4 1990





// upgrade gen3->gen4, using probability 1 in 4
random_switch(FEAT_, SELF, randup4, bitmask(TRIGGER_VEHICLE_SERVICE)){
4:generation3;
}
// upgrade gen3->gen4 available?
switch(FEAT_, SELF, testup4, current_year){
0..(YEAR_GEN4-1):generation3;
default:randup4;
}
// downgrade gen4->gen3 to fix initial randomness
random_switch(FEAT_, SELF, randdown4, bitmask(TRIGGER_VEHICLE_SERVICE)){
dependent: randup4;
4:generation3;
}
// gen4 already available?
switch(FEAT_, SELF, testdown4, current_year){
0..(YEAR_GEN4-1):randdown4;
default:generation4;
}
// at least gen3, already upgraded to gen4?
random_switch(FEAT_, SELF, maybegen4){
dependent: randup4;
3:testup4;
1:testdown4;
}



// upgrade gen2->gen3, using probability 1 in 4
random_switch(FEAT_, SELF, randup3, bitmask(TRIGGER_VEHICLE_SERVICE)){
4:generation2;
}
// upgrade gen2->gen3 available?
switch(FEAT_, SELF, testup3, current_year){
0..(YEAR_GEN3-1):generation2;
default:randup3;
}
// downgrade gen3->gen2 to fix initial randomness
random_switch(FEAT_, SELF, randdown3, bitmask(TRIGGER_VEHICLE_SERVICE)){
dependent: randup3;
4:generation2;
}
// gen3 already available?
switch(FEAT_, SELF, testdown3, current_year){
0..(YEAR_GEN3-1):randdown3;
default:maybegen4;
}
// at least gen2, already upgraded to gen3?
random_switch(FEAT_, SELF, maybegen3){
dependent: randup3;
3:testup3;
1:testdown3;
}



// upgrade gen1->gen2, using probability 1 in 4
random_switch(FEAT_, SELF, randup2, bitmask(TRIGGER_VEHICLE_SERVICE)){
4:generation1;
}
// upgrade gen1->gen2 available?
switch(FEAT_, SELF, testup2, current_year){
0..(YEAR_GEN2-1):generation1;
default:randup2;
}
// downgrade gen2->gen1 to fix initial randomness
random_switch(FEAT_, SELF, randdown2, bitmask(TRIGGER_VEHICLE_SERVICE)){
dependent: randup2;
4:generation1;
}
// gen2 already available?
switch(FEAT_, SELF, testdown2, current_year){
0..(YEAR_GEN2-1):randdown2;
default:maybegen3;
}
// at least gen1, already upgraded to gen2?
random_switch(FEAT_, SELF, maybegen2){
dependent: randup2;
3:testup2;
1:testdown2;
}



// default generation by build-year
switch(FEAT_, SELF, main_graphics, build_year){
0..(YEAR_GEN2-1): maybegen2
YEAR_GEN2..(YEAR_GEN3-1): maybegen3
YEAR_GEN3..(YEAR_GEN4-1): maybegen4
default: generation4;
}