Fix handling of () used as an expression
Handle block conditions
Generate skipped test for non-parser ParseTree test cases
Specify a list of ParseTree tests to skip

For starters, there is only one test on this list
Create #safe_wrap_in_block and use it when returning from process_x methods

The process_x methods must never return nil. Therefore, #wrap_in_block is
potentially unsafe as it sometimes returns nil. #safe_wrap_in_block will instead
return the emtpy Sexp s().
Properly handle empty else block in case..when
Handle ctrl+meta escape sequence
Convert :dot2 to :flip2 in conditionals
handle empty ensure blocks
Handle assignement of exceptions to instance variables