0 If something is optional, you can choose if you want to do it, pay it, buy it, etc. -- 可選擇的；非強制的
English is compulsory for all students, but art and music are optional. 英語是所有學生的必修課，而藝術和音樂則是選修的。
Seat-belts are included as standard but electric windows are an optional extra.
There are several optional tours, for which there will be a small charge.
The basic model is quite cheap, but there are a lot of optional extras.
We show how to extend any existing parser combinator library with support for parsing permutations of typed, potentially optional elements.
The final (optional) element of the sequence gives the values of the syntactic functions, adjuncts and specifiers (in any order, distinguished by their role names).
Take is usually a transitive verb, yet although it has an obligatory direct-object argument, locative arguments are normally optional.
