Allow any form field to be represented by reverse arc in the graph

All the form fields at the moment assume that they are given a subject X and a predicate P and the form will write a quad (X, P Y, G).

In fact, when not dealing with literals for Y, RDF is symmetrical: it is just as reasonable for them to correspond to the data (Y, P, X, G). So we need to be able to specify in a form, that a given field is in the inverse direction.

Examples:

• When a user is a member of a group, the triple (user, member, group) could turn up in a form about the user - or a form about the group.

• In a family tree user interface form, the same form could allow adding a number of (this, child, Y) triples to list one's children, and also a set of (Y, child, this) triples for giving ones parents.

Start by tracing the TypeScript form-field implementation that currently writes subject–predicate–object quads, and inspect how fields are configured and tested. Define how a field selects the inverse direction, then verify that both forward and reverse relationships can be displayed and written for the membership and family-tree examples.