"Rootbeer was created using Test Driven Development and testing is essentially important in Rootbeer"

I'm not sure what the "essentially" means here, but this is the first "big" program I'm aware of that name-checks TDD, and a counter-example to my theory that programs where much of the programmer effort goes into algorithms and data structures are not suited to TDD.

Was the TDD approach "pure"? (Only one feature implemented at a time, with absolutely no design thought given to what language features might need to be implemented in the future.)