WAC editor: replace the resource in the .meta graph with a new one. #4

As it is, using the WAC editor only appends triples to the graph. This means that you cannot remove an existing triple. Example:

For a given WAC rule:

<#dir>
    <acl#accessTo> <dir> ;
    <acl#agent> <webid#1>, <webid#2> ;
    <acl#mode> <acl#Read>, <acl#Write> .

Currently, removing the Write mode from rule <#dir> means doing an HTTP POST with the following turtle data:

<#dir>
    <acl#accessTo> <dir> ;
    <acl#agent> <webid#1>, <webid#2> ;
    <acl#mode> <acl#Read> .

However, since POST only performs an append, the triple <#dir> <acl#mode> <acl#Write> will not be removed.