declare WhyML keywords at a unique location
There are currently several files where WhyML keywords are used, and they are not clearly synchronised, even with the comment at the beginning of file src/parser/lexer.mll
. Moreover, it seems that the file src/core/pretty.ml
should have been mentioned in the former comment as one file to keep in sync.
It would be better to factorise the implementation and put the keywords at only one place