How does the <? coalescing minimum operator work in EK9?

defines module qa.operators.coalescingmin

  defines function

    <?-
      Find the cheaper price using <? coalescing minimum.
      If either price is unset, returns the other.
    -?>
    bestDeal() as pure
      ->
        onlinePrice as Float
        storePrice as Float
      <-
        rtn as Float: onlinePrice <? storePrice

    <?-
      Find the closest deadline using <? coalescing minimum on dates.
      Works with any type that has the <=> comparison operator.
    -?>
    nearestDeadline() as pure
      ->
        deadlineA as Date
        deadlineB as Date
      <-
        rtn as Date: deadlineA <? deadlineB

    maxOf() as pure
      ->
        left as Integer
        right as Integer
      <- rtn as Integer: left >? right

    chainedMinFloat() as pure
      ->
        a as Float
        b as Float
        c as Float
      <- rtn as Float: a <? b <? c

  defines program

    CoalescingMinDemo()
      stdout <- Stdout()

      // Both values set — returns the smaller
      cheapest <- bestDeal(29.99, 19.99)
      stdout.println(`Cheapest: ${cheapest}`)

      // One value unset — returns the SET one
      knownPrice <- 25.0
      unknownPrice <- Float()
      safePrice <- knownPrice <? unknownPrice
      stdout.println(`Safe price: ${safePrice}`)

      // Both unset — result is unset
      noPrice1 <- Float()
      noPrice2 <- Float()
      noResult <- noPrice1 <? noPrice2
      if noResult?
        stdout.println("Should not reach here")
      else
        stdout.println("Both unset, result is unset")

      // >? coalescing maximum — returns the larger
      topScore <- maxOf(85, 92)
      stdout.println(`Top score: ${topScore}`)

      // Chaining: find minimum of three values
      lowest <- chainedMinFloat(30.0, 15.0, 22.0)
      stdout.println(`Lowest of three: ${lowest}`)

min(onlinePrice, storePrice)

onlinePrice <? storePrice