second iteration starts
Test: 0
TPP: nil -> constant
Test: 1 and 2. Finally refactored to one test named "single digit convert to one word"
TPP: constant -> array ("zero" -> WORDS[number])
Test: add the rest cases (3..9) for single digit test
TPP: constant -> constant+ (adding more constant elements into WORDS array)
In fact, I'm not sure we should categorize this transformation as such. A transformation to add more elements into an array could be categorized as:
1. constant -> constant+
2. statement -> statements
3. a new transformation not in current TPP list
Test: two digits until 20 convert to one word (10..20)
TPP: constant -> constant+ (same as previous TPP)