anchor.diff:  * `Enum_optional` - presents a value of a designated type (see field `Field_typeref`) as a dropdown list for edting, or as a hyperlink for viewing.  The value may be left unselected, in which case the stored value is blank.
   * `Field` - identifies a field description; subsumed by `Enum_choice`.
   * `FileUpload` - upload file as resource attached to entity.  Stored as a complex structure with filename, resource reference, content type, etc.;  presented for editing as an HTML file upload input element, and for viewing as a hyperlink.  See section "Resource references, imports and file uploads"
  -* `Identifier` - a [CURIE](http://www.w3.org/TR/curie/) or [URI](https://tools.ietf.org/html/rfc3986).  Preesented for editing as a text box, and for viewing as a simple text element.
  +* `Identifier` - a http://www.w3.org/TR/curie/[CURIE] or https://tools.ietf.org/html/rfc3986[URI].  Preesented for editing as a text box, and for viewing as a simple text element.
   * `List` - identifies a list description; subsumed by `Enum_choice`.
   * `Markdown` - multiline rich text.  Stored and presented for editing as Markdown text, and for viewing as  text formatted according to Markdown conventions.
   * `Placement` - a special-case field renderer used for presenting placement of a field on a form.
  
positive.code: 
  * Reference to linked entity (`Value_entity`), used with `RefMultifield` render type: the displayed value is one or more fields selected from a linked entity (e.g., a reference to an entity record containing an image and description can be displayed as image and descripotion fields from that record.)  When editing, a selection is made from a dropdown list of available entities.
  
  ### Position/size (`Field_placement`)
  
  Used to specifying the position of of a field in a form display, specified in terms of width and horizontal placement on a responsive display grid <sup>1,2</sup>.
  
  
negative.code: 
  These different cases are invoked as follows.
  
  === Direct reference to a resource (usually an external resource)
  
  Field "Field value type" (`Field_type`) describes the target value (e.g. `annal:Identifier` or `annal:Image`), and field "Enum type" (`Field_typeref`) is unspecified or blank.
  
  In this case the field value is used directly as the resource URI, and for editing is presented as a text input box for the URI.
  
  === Reference to an imported or uploaded attachment in a designated entity
  
  The value of field "Enum type" (`Field_typeref`) is the target entity type, and field "Referenced field" (`Field_fieldref`) is target field property CURIE or URI for the attachment; this is the same as the "Property" (`Field_property`) value in the target field description.
  
  The stored field value is an identifier for a selected target record, and is presented for editing as a drop-down list of entity identifiers.
  
  === Reference to an imported attachment in the current entity
  
  Field "Field value type" (`Field_type`) is `annal:Import`, and field "Enum type" (`Field_typeref`) is unspecified or blank.
  
  In this case the field value describes an attachment to the current entity, and for editing is presented as render type `annal:URIImport`.
  
  === Reference to an uploaded attachment in the current entity
  
  (@@TODO: not yet implemented)
  
  