L-Systems: Order of Substitution

Question

I posted this over at the mathematics stack excahnge, but as its programming related I wondered if somebody can help out here.

I am working through a subject guide on involving L-Systems and have the following alphabet A = {a, b, c}. The initiator is the string "a" and the rules of substitution a → ba, b → ccb, c → a.

The study guide gives the first five generations as:

[a] → [ba] → [ccba] → [acba] → [aaba] → [aaccba]

I can't for the life of me figure out how this works. No rules regarding the order of substitution are provided, and my lecturer say's that it is possible to get to this.

Does anybody have any ideas?