instance_id[0 ... n];
This read only array holds all the ids of every active
instance within the room. This means that if you have used any of
functions those instances that have been deactivated will not be
included in this array (if you have used a value from this array
previously, it will now return the keyword noone).
for (i = 0; i < instance_count; i += 1;)
with (instance_id[i]) speed += 0.1;
The above code will loop through all instances within the room and add 0.1 to their speed.