Mrmime.FieldSourcetype 'a t = | Date : Date.t t| Mailboxes : Mailbox.t list t| Mailbox : Mailbox.t t| Addresses : Address.t list t| MessageID : MessageID.t t| Unstructured : Unstructured.t t| Unstructured_with_encoded : Unstructured_with_encoded.t t| Content : Content_type.t t| Encoding : Content_encoding.t tType of kind of values according RFC2045/RFC5322.
*)make field_name w v returns a field.
Pretty-printer of field.