code: function _groupContribs (contribs) {
    let groups = new Map()
    groups.set('NOGROUP', [])
    for (let contrib of contribs) {
      let groupId = contrib.group
      if (groupId) {
        if (!groups.has(groupId)) {
          groups.set(groupId, [])
        }
        groups.get(groupId).push(contrib)
      } else {
        groups.get('NOGROUP').push(contrib)
      }
    }
    return groups
  }
docs: /*
  Uses group association of person nodes to create groups
  
  [p1,p2g1,p3g2,p4g1] => {p1: p1, g1: [p2,p4], g2: [p3] }