transaction_command

transaction_command 
      ::= 
      ( "commit" [ "work" ] [ ( "comment" quoted_string ) 
      | ( "force" quoted_string [ "," n ] ) ] ) 
      | ( "set" "transaction" "read" "only" ) 
      | ( "savepoint" savepoint_name ) 
      | ( "rollback" [ "work" ] [ ( "to" [ "savepoint" savepoint_name ] ) 
      | ( "force" quoted_string ) ] ) 

This rule is called by

to_modify_data