code: function onlyCircularLink (link) {
      var nodeSourceLinks = link.source.sourceLinks
      var sourceCount = 0
      nodeSourceLinks.forEach(function (l) {
        sourceCount = l.circular ? sourceCount + 1 : sourceCount
      })
  
      var nodeTargetLinks = link.target.targetLinks
      var targetCount = 0
      nodeTargetLinks.forEach(function (l) {
        targetCount = l.circular ? targetCount + 1 : targetCount
      })
  
      if (sourceCount > 1 || targetCount > 1) {
        return false
      } else {
        return true
      }
    }
docs: Check if a circular link is the only circular link for both its source and target node