flatten arrow code

[hchiam]

For my case, from https://blog.codinghorror.com/flattening-arrow-code/ I'll try:

• use "guard conditions" (shorter negative block before the longer positive one)

For my case, from https://softwareengineering.stackexchange.com/questions/205803/how-to-tackle-a-branched-arrow-head-anti-pattern I'll try:

• create "composed" boolean checks to make nested if-statements into a linear list of if-statements
• use returns for mutually-exclusive cases
• (and less likely: otherwise don't use returns, and put them before the cases that return)

[hchiam]

v0.1.0 Interpreter --> v0.2.0 Transformer (much flatter)