==== SSE

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

+ test another form -- (debug FOO)

+ Expr: (exists (pattern ...))
  
+ (group (vars) aggregate)
  
+  Assignment:
  (assign ((?x expr) (?y expr)) --  (:= ?x expr) [is this SELECT/project?]
     (project ...))
  VarExprLists      

+ Start a collection of Writers to go with Builders.
  Move out OpWriter , break up into pieces.