content = mixed { (persName | eventName)* }
nonEmpty = xsd:string { minLength = "1" }
persName = element persName { ref, nonEmpty }
ref =
    attribute ref {
        xsd:string { pattern = "#\S+" }
    }
eventName = element eventName { ref, nonEmpty }
start = letter
letter = element letter { header, \text }
header = element header { title, provenance }
provenance = element provenance { nonEmpty }
title = element title { nonEmpty }
\text = element text { date, salutation, body, closing }
date = element date { xsd:date }
salutation = element salutation { content }
body = element body { p+ }
p = element p { content }
closing = element closing { valediction?, signature }
valediction = element valediction { content }
signature = element signature { content }