To enable the Stacks 3.5 preprocessor and disable old-style preprocssor set this key to YES. It is strongly recommended that you use enable the new preprocessor as it is significantly faster.
NB: Opting in to the new-style preprocessor removes support for all deprecated API.
When the preprocessor is enabled conditional mode macros will have a new format and may not be nested.
The new format doubles the percent signs and square-brackets.