remove debugging codes
Merge branch 'master' into eopl
ex 5.5 - grammar def & testcases
ex 5.5 - emptylist and cons
ex 5.5 - done
ex 5.6 - done
ex 4.18 - done
ex 4.19 - done
ex 5.7 - done
ex 5.1 - rethink of this exercise

the previous solution might be wrong:
the procedual representation should really be procedures
ex 5.1 - alternative solution done
ex 5.1 - rearrange dir
ex 5.1 - rearrange & done
ex 4.20 - get started
ex 4.20 - done transformation, need some tests
ex 5.8 - done
ex 5.9 - stuffs necessary
ex 5.9 - use proc-rep should be much more easier
ex 5.9 - procedural approach done
ex 5.9 - move dir