anchor.diff:  
   In this case the field value describes an attachment to the current entity, and for editing is presented as render type `annal:FileUpload`.
   
  -### Multi-field references
  +=== Multi-field references
   
   @@TODO
   
   
  -## Field render types
  +== Field render types
   
   Annalist provides a number of built-in field rendering functions for dealing with different types and uses of field data.
   
  
positive.code: * `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.
  * `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.
  
negative.code: 
  Default placement can be specified as part of the field description, and overridden when the field is included in a particular view.
  
  === Placeholder (`Field_placeholder`)
  
  A string that is presented to describe the expected field content when the field content is empty
  
  === Property (`Field_property`)
  
  A http://www.w3.org/TR/curie/[CURIE] or https://tools.ietf.org/html/rfc3986[URI] that is used to relate the field value to the containing entity.  The supplied string is used as a key value in the stored JSON.  The use of CURIE or URI formats for this key allows Annalist data to be interpreted as JSON-LD, hence as http://linkeddata.org[linked data].
  
  A default property CURIE or URI can be specified as part of the field description, and overridden when the field is included in a particular view.
  
  (@@TODO: not yet implemented is management of prefix URIs and JSON-LD contexts required to fully support use as linked data)
  
  === Default (`Field_default`)
  
  A default value for the field if none is specified.
  
  === Enum type (`Field_typeref`)
  
  Used with render types `Enum`, `Enum_optional` and `Enum_choice` (and also `Type`, `List`, `View`, `Field` which are sumsumed by the `Enum*` render types).
  
  When specified, this field value is an internal type identifier.  The field is taken to be a reference to an entity of the given type, and presented for editing as a drop-down list of available values.  For viewing, the field is presented as a hyperlink to a description of the corresponding type.
  
  === Referenced field (`Field_fieldref`)
  
  When a field refers to some target entity, this may indicate a property CURIE or URI for a field of that entity that is used for view rendering.
  
  See section "Resource references, imports and file uploads" for more details.
  
  === Field group (`Field_groupref`)
  
  Field group reference used by `RepeatGroup`, `RepeatGroupRow` and `RefMultifield` renderers.  Otherwise, it is ignored.
  
  