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diff --git a/static/ts/types.ts b/static/ts/types.ts
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+/**
+ * Collection of type information for calendar data.
+ * @module
+ */
+
+export {
+    ical_type,
+    valid_input_types,
+    JCalProperty, JCal,
+    xcal, uid,
+    ChangeLogEntry
+}
+
+
+/** Name of all valid icalendar types. */
+let all_types = [
+    'text',
+    'uri',
+    'binary',
+    'float',        /* Number.type = 'float' */
+    'integer',      /* Number.type = 'integer' */
+    'date-time',    /* Date */
+    'date',         /* Date.dateonly = true */
+    'duration',     /* TODO */
+    'period',       /* TODO */
+    'utc-offset',   /* TODO */
+    'cal-address',
+    'recur',        /* RRule */
+    'boolean',      /* boolean */
+]
+
+
+/** The union of all elements in `all_types`. */
+type ical_type
+    = 'text'
+    | 'uri'
+    | 'binary'
+    | 'float'
+    | 'integer'
+    | 'date-time'
+    | 'date'
+    | 'duration'
+    | 'period'
+    | 'utc-offset'
+    | 'cal-address'
+    | 'recur'
+    | 'boolean'
+    | 'unknown'
+
+/**
+ * All known names properties (top level keys) can have.
+ * Such as "calscale", "dtstart", ...
+ */
+let property_names = [
+    'calscale', 'method', 'prodid', 'version', 'attach', 'categories',
+    'class', 'comment', 'description', 'geo', 'location', 'percent-complete',
+    'priority', 'resources', 'status', 'summary', 'completed', 'dtend', 'due',
+    'dtstart', 'duration', 'freebusy', 'transp', 'tzid', 'tzname', 'tzoffsetfrom',
+    'tzoffsetto', 'tzurl', 'attendee', 'contact', 'organizer', 'recurrence-id',
+    'related-to', 'url', 'uid', 'exdate', 'exrule', 'rdate', 'rrule', 'action',
+    'repeat', 'trigger', 'created', 'dtstamp', 'last-modified', 'sequence', 'request-status'
+];
+
+
+/**
+ * Which property fields each component can hold.
+ *
+ * ```json
+ * { 'VCALENDAR': ['PRODID', 'VERSION', 'CALSCALE', 'METHOD'],
+ *     ...
+ * }
+ * ```
+ */
+let valid_fields: Map<string, string[]> = new Map([
+    ['VCALENDAR', ['PRODID', 'VERSION', 'CALSCALE', 'METHOD']],
+    ['VEVENT', ['DTSTAMP', 'UID', 'DTSTART', 'CLASS', 'CREATED',
+        'DESCRIPTION', 'GEO', 'LAST-MODIFIED', 'LOCATION',
+        'ORGANIZER', 'PRIORITY', 'SEQUENCE', 'STATUS',
+        'SUMMARY', 'TRANSP', 'URL', 'RECURRENCE-ID',
+        'RRULE', 'DTEND', 'DURATION', 'ATTACH', 'ATTENDEE',
+        'CATEGORIES', 'COMMENT', 'CONTACT', 'EXDATE',
+        'REQUEST-STATUS', 'RELATED-TO', 'RESOURCES', 'RDATE']],
+    ['VTODO', ['DTSTAMP', 'UID', 'CLASS', 'COMPLETED', 'CREATED',
+        'DESCRIPTION', 'DTSTART', 'GEO', 'LAST-MODIFIED',
+        'LOCATION', 'ORGANIZER', 'PERCENT-COMPLETE', 'PRIORITY',
+        'RECURRENCE-ID', 'SEQUENCE', 'STATUS', 'SUMMARY', 'URL',
+        'RRULE', 'DUE', 'DURATION', 'ATTACH', 'ATTENDEE', 'CATEGORIES',
+        'COMMENT', 'CONTACT', 'EXDATE', 'REQUEST-STATUS', 'RELATED-TO',
+        'RESOURCES', 'RDATE',]],
+    ['VJOURNAL', ['DTSTAMP', 'UID', 'CLASS', 'CREATED', 'DTSTART', 'LAST-MODIFIED',
+        'ORGANIZER', 'RECURRENCE-ID', 'SEQUENCE', 'STATUS', 'SUMMARY',
+        'URL', 'RRULE', 'ATTACH', 'ATTENDEE', 'CATEGORIES', 'COMMENT',
+        'CONTACT', 'DESCRIPTION', 'EXDATE', 'RELATED-TO', 'RDATE',
+        'REQUEST-STATUS']],
+    ['VFREEBUSY', ['DTSTAMP', 'UID', 'CONTACT', 'DTSTART', 'DTEND',
+        'ORGANIZER', 'URL', 'ATTENDEE', 'COMMENT', 'FREEBUSY',
+        'REQUEST-STATUS']],
+    ['VTIMEZONE', ['TZID', 'LAST-MODIFIED', 'TZURL']],
+    ['VALARM', ['ACTION', 'TRIGGER', 'DURATION', 'REPEAT', 'ATTACH',
+        'DESCRIPTION', 'SUMMARY', 'ATTENDEE']],
+    ['STANDARD', ['DTSTART', 'TZOFFSETFROM', 'TZOFFSETTO', 'RRULE',
+        'COMMENT', 'RDATE', 'TZNAME']],
+])
+
+valid_fields.set('DAYLIGHT', valid_fields.get('STANDARD')!);
+
+/**
+   All registered property types for VComponents.
+
+   Note that only some of these are valid for each type of component (VCALENDAR,
+   VEVENT, ...), and that they all support both iana and `x-` extensions.
+ */
+type known_ical_types
+    = 'ACTION'
+    | 'ATTACH'
+    | 'ATTENDEE'
+    | 'CALSCALE'
+    | 'CATEGORIES'
+    | 'CLASS'
+    | 'COMMENT'
+    | 'COMPLETED'
+    | 'CONTACT'
+    | 'CREATED'
+    | 'DESCRIPTION'
+    | 'DTEND'
+    | 'DTSTAMP'
+    | 'DTSTART'
+    | 'DUE'
+    | 'DURATION'
+    | 'EXDATE'
+    | 'FREEBUSY'
+    | 'GEO'
+    | 'LAST-MODIFIED'
+    | 'LOCATION'
+    | 'METHOD'
+    | 'ORGANIZER'
+    | 'PERCENT-COMPLETE'
+    | 'PRIORITY'
+    | 'PRODID'
+    | 'RDATE'
+    | 'RECURRENCE-ID'
+    | 'RELATED-TO'
+    | 'REPEAT'
+    | 'REQUEST-STATUS'
+    | 'RESOURCES'
+    | 'RRULE'
+    | 'SEQUENCE'
+    | 'STATUS'
+    | 'SUMMARY'
+    | 'TRANSP'
+    | 'TRIGGER'
+    | 'TZID'
+    | 'TZNAME'
+    | 'TZOFFSETFROM'
+    | 'TZOFFSETTO'
+    | 'TZURL'
+    | 'URL'
+    | 'VERSION'
+
+/**
+ * Which types are valid to store under each property.
+ * If multiple values are an option for that property, then
+ * the list of possibilities will contain a sub-list (see example).
+ *
+ * ```json
+ * { 'DTSTART': ['date', 'date-time'],
+ *   'CATEGORIES': [['text']],
+ *   ...
+ * }
+ * ```
+ */
+let valid_input_types: Map<string, Array<ical_type | ical_type[]>> =
+    new Map([
+        ['ACTION', ['text']], // AUDIO|DISPLAY|EMAIL|*other*
+        ['ATTACH', ['uri', 'binary']],
+        ['ATTENDEE', ['cal-address']],
+        ['CALSCALE', ['text']],
+        ['CATEGORIES', [['text']]],
+        ['CLASS', ['text']], // PUBLIC|PRIVATE|CONFIDENTIAL|*other*
+        ['COMMENT', ['text']],
+        ['COMPLETED', ['date-time']],
+        ['CONTACT', ['text']],
+        ['CREATED', ['date-time']],
+        ['DESCRIPTION', ['text']],
+        ['DTEND', ['date', 'date-time']],
+        ['DTSTAMP', ['date-time']],
+        ['DTSTART', ['date', 'date-time']],
+        ['DUE', ['date', 'date-time']],
+        ['DURATION', ['duration']],
+        ['EXDATE', [['date', 'date-time']]],
+        ['EXRULE', []], /* deprecated */
+        ['FREEBUSY', [['period']]],
+        ['GEO', ['float']], // pair of floats
+        ['LAST-MODIFIED', ['date-time']],
+        ['LOCATION', ['text']],
+        ['METHOD', ['text']],
+        ['ORGANIZER', ['cal-address']],
+        ['PERCENT-COMPLETE', ['integer']], // 0-100
+        ['PRIORITY', ['integer']], // 0-9
+        ['PRODID', ['text']],
+        ['RDATE', [['date', 'date-time', 'period']]],
+        ['RECURRENCE-ID', ['date', 'date-time']],
+        ['RELATED-TO', ['text']],
+        ['REPEAT', ['integer']],
+        ['REQUEST-STATUS', ['text']],
+        ['RESOURCES', [['text']]],
+        ['RRULE', ['recur']],
+        ['SEQUENCE', ['integer']],
+        ['STATUS', ['text']], // see 3.8.1.11
+        ['SUMMARY', ['text']],
+        ['TRANSP', ['text']], // OPAQUE|TRANSPARENT
+        ['TRIGGER', ['duration', 'date-time']],
+        ['TZID', ['text']],
+        ['TZNAME', ['text']],
+        ['TZOFFSETFROM', ['utc-offset']],
+        ['TZOFFSETTO', ['utc-offset']],
+        ['TZURL', ['uri']],
+        ['UID', ['text']],
+        ['URL', ['uri']],
+        ['VERSION', ['text']],
+    ])
+
+// type JCalLine {
+// }
+
+/** The UID of a VEvent, to make declarations clearer.  */
+type uid = string
+
+/* TODO is this type correct?
+   What really are valid values for any? Does that depend on ical_type? Why is the tail a list?
+   What really is the type for the parameter map?
+*/
+/* TODO link to RFC 7265 (jCal) */
+/**
+ * Alias for a record consisting of
+ * - the name of the type, as a string
+ * - all parameters of the object, as a `Record<String, any>`
+ * - An `ical_type` value, noting the type of the final field(s)
+ * - and one or more values of the type specified by the third field.
+ */
+type JCalProperty
+    = [string, Record<string, any>, ical_type, any]
+    | [string, Record<string, any>, ical_type, ...any[]]
+
+/**
+   Base type for JCal documents.
+
+   Each VComponent in a JCal document is of this form.
+
+   - The first element is the components type
+     ('vevent', 'vcalendar', ...), in all lower case
+   - The second element is is all properties directly
+     on the component.
+   - The third element is a list of all children.
+*/
+type JCal = [string, JCalProperty[], JCal[]]
+
+/** The xml namespace name for xcalendar */
+const xcal = "urn:ietf:params:xml:ns:icalendar-2.0";
+
+/**
+   An entry into a changelog.
+
+   This is primarily used by VEvent, to track what has happened during a
+   session.
+ */
+interface ChangeLogEntry {
+    /**
+       Type of change
+
+       'property' is used for changes to properties.
+
+       'calendar' is used when the containing calendar of a VEVENT is changed
+    */
+    type: 'calendar' | 'property',
+    /** The name of the filed changed */
+    name: string,
+    /** The previous value, `null` if just created */
+    from: string | null,
+    /** The new value, `null` if removed */
+    to: string | null,
+}