Ability to directly edit relationships (specificall add/remove NOT)

[hdrabkin]

This came up yesterday working on this model:
http://noctua.geneontology.org/workbench/noctua-form/?model_id=gomodel%3AMGI_MGI_1195268
This had 6 Protein Ontology (PRO) isoforms coded for by the gene. Only the full length isoform 1 had an activity, the others were shown to not have the activity.
After adding one NOT annotation for isoform 2, I then added the annotations for isoform 3-6, but forgot to add the NOT at the time of creating the annotation. After realizing, I discovered that I could not find a way to go back and add the NOT .
I could use the graph view to find and add the NOT to the enabled by for the isoform activities, and then remove the relationship that was not a NOT.
However, it would have been desirable to just do this in the form without having to delete all the incorrect annotations completely and redo them using the NOT David, Li, and I would find no way to do this.