anchor.diff:     ;
   
   explain_option_list
  -   : explain_option_elem
  -   | explain_option_list COMMA explain_option_elem
  +   : explain_option_elem (COMMA explain_option_elem)*
      ;
   
   explain_option_elem
  
positive.code:    ;
  
  overlay_list
     : a_expr PLACING a_expr FROM a_expr FOR a_expr
     | a_expr PLACING a_expr FROM a_expr
     ;
  
  position_list
  
negative.code:    ;
  
  frame_bound
     : UNBOUNDED (PRECEDING | FOLLOWING)
     | CURRENT_P ROW
     | a_expr PRECEDING
     | a_expr FOLLOWING
     ;
  
  opt_window_exclusion_clause
     : EXCLUDE (CURRENT_P ROW | GROUP_P | TIES | NO OTHERS)
     |
     ;
  
  row
     : ROW OPEN_PAREN expr_list? CLOSE_PAREN
     | OPEN_PAREN expr_list COMMA a_expr CLOSE_PAREN
     ;
  
  