src/Router.js
function backTo(stateName, defaultParams, acc) {
    const params = leafStates[stateName].lastParams || defaultParams
    transitionTo(stateName, params, acc)
  }
function backTo(stateName, defaultParams, acc) {
    const params = leafStates[stateName].lastParams || defaultParams
    transitionTo(stateName, params, acc)
  }
/*
Attempt to navigate to 'stateName' with its previous params or
fallback to the defaultParams parameter if the state was never entered.