anchor.diff:   existence_test
    subquery
    scalar_exp
  + scalar_sub_exp
    scalar_exp_commalist
    atom
    parameter_ref
  
positive.code: 
      %% operators
  
  Left        100 'OR'.
  Left        110 'AND'.
  Left        120 'NOT'.
  Nonassoc    200 COMPARISON. %% = <> < > <= >=
  Left        300 '+' '-'.
  Left        400 '*' '/'.
  
negative.code: assignment_commalist -> assignment_commalist ',' assignment                                     : '$1' ++ ['$3'].
  
  assignment -> column '=' scalar_exp                                                             : {'=', '$1', '$3'}.
  
  update_statement_searched -> UPDATE table SET assignment_commalist opt_where_clause             : {'update', '$2', {'set', '$4'}, '$5'}.
  
  