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;
}

// at least gen3, already upgraded to gen4?
random_switch(FEAT_, SELF, maybegen4){
dependent: randup4;
3:testup4;
1:generation4;
}



// 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;
}

// at least gen2, already upgraded to gen3?
random_switch(FEAT_, SELF, maybegen3){
dependent: randup3;
3:testup3;
1:maybegen4;
}



// 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;
}

// at least gen1, already upgraded to gen2?
random_switch(FEAT_, SELF, maybegen2){
dependent: randup2;
3:testup2;
1:maybegen3;
}



// 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;
}