To achieve a similar functionality as conditional layers, you just do the following:
on base layer:
- key a - switch to layer a
- key b - switch to layer b
on layer a:
- key b - switch to layer c
on layer b:
- key a - switch to layer c
Thus, if you press a you get layer a; if you press b you get layer b; if you press a followed by b you get layer c; if you press b followed by a you get layer c.
Of course when you let go of all keys you end up back at the base layer.