anchor.diff:  bool_lit: TRUE | FALSE
       ;
   
  -function_call: 
  +function_call:
                priority? f=ID a=expr_argument_list -> ^( CALL_FUNCTION $f $a priority?)
       ;
   
  
positive.code: // only allow function calls as non-assignment statements
  expr_stmt: e=function_call -> ^( EXPR_STMT $e )
      ;
      
  update_stmt: v=ID LT cmd=ID GT MUTATE expr SEMICOLON
  			-> ^( UPDATE $cmd $v expr)
  	;
  
negative.code: 
  
  stmt:
      SEMICOLON ->
      |   (declaration_multi)
      |   (assignment_expr)
      |   (expr_stmt)         SEMICOLON -> expr_stmt
      |   (if_stmt)
  