Check for fhm according to nfp, not just temp shift

lib/sympto/index.js

@@ -4,19 +4,18 @@ import getPreOvulatoryPhase from './pre-ovulatory'
 import { LocalDate } from 'js-joda'
 import assert from 'assert'
 
-export default function ({ cycle, previousCycles = [] }) {
+export default function getSymptoThermalStatus({ cycle, previousCycles = [] }) {
   throwIfArgsAreNotInRequiredFormat(cycle, previousCycles)
 
   const status = {
     assumeFertility: true,
     phases: {}
   }
-  // TODO handle no previous cycles
 
   // if there was no first higher measurement in the previous cycle,
   // no infertile pre-ovulatory phase may be assumed
   const lastCycle = previousCycles[previousCycles.length - 1]
-  if (getTemperatureShift(lastCycle).detected && !cycle[0].mucus) {
+  if (lastCycle && getSymptoThermalStatus({cycle: lastCycle}).temperatureShift) {
     status.phases.preOvulatory = getPreOvulatoryPhase(cycle, previousCycles)
     if (status.phases.preOvulatory.cycleDays.length === cycle.length) {
       status.assumeFertility = false