The [Capn Proto FAQ](
https://capnproto.org/faq.html#how-do-i-make-a-field-required-like-in-protocol-buffers) has a rant about the required field that elucidates (in part) the history behind it.
Note also that both Capn Proto and Protocol Buffers 3 still support the notion of optional - Capn Proto via [unions](
https://capnproto.org/faq.html#how-do-i-make-a-field-optional) and Protocol Buffers 3 via [OneOf](
https://developers.google.com/protocol-buffers/docs/proto3#oneof).
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