==== SSE

+ Comment character of # is too valuable.
  Use '--', ';;', '//', '/* */', '{{', '~~'
  ## is heavy. '%'  
  (comment !!!!)

+ Use of {} for ???

+ Literals and @
  Include @ into literal terminator 
  Currently, symbols can't start with a @ or ^ 

+ Reader-writer tests

+ Expr: (exists (pattern ...))
  Tidy expr writing - consolidate in WriterExpr
  
+ PrintSerializable does not really work because it assumes one serialization form.
  OK for human readable but that's about it.
  Possible remove "output(, SerializationContext)" form and leave as support for toString.