code: function parents (pipe, taskKey) {
    var inputPipesOf = inputPipes.bind(null, pipe)
    var parentTaskIds = []
  
    function pushParentTaskId (pipe) {
      parentTaskIds.push(pipe[0])
    }
  
    inputPipesOf(taskKey).forEach(pushParentTaskId)
  
    return parentTaskIds
  }
docs: Compute parent tasks.
  
  @param {Array} pipes of graph
  @param {String} taskKey
  
  @returns {Array} parentTaskIds