OCaml record field.
| Field of ('perm, 'record, 'field) For_generated_code.t
'record is the type of the record.
'field is the type of the
values stored in the record field with name
'perm is a way
of restricting the operations that can be used.
A record field with no restriction.
A record that can only be read, because it belongs to a private type.