Make ClockMark an abstract class instead of an interface

libraries/stdlib/src/kotlin/time/Clock.kt

@@ -24,29 +24,29 @@ public interface Clock {
  * Represents a time point notched on a particular [Clock]. Remains bound to the clock it was taken from
  * and allows querying for the duration of time elapsed from that point (see the function [elapsed]).
  */
-public interface ClockMark {
+public abstract class ClockMark {
     /**
      * Returns the amount of time passed from this clock mark on the clock from which this mark was taken.
      */
-    fun elapsed(): Duration
+    abstract fun elapsed(): Duration
 
     /**
      * Returns a clock mark on the same clock that is ahead of this clock mark by the specified [duration].
      *
      * The returned clock mark is more _late_ when the [duration] is positive, and more _early_ when the [duration] is negative.
      */
-    operator fun plus(duration: Duration): ClockMark = AdjustedClockMark(this, duration)
+    open operator fun plus(duration: Duration): ClockMark = AdjustedClockMark(this, duration)
 
     /**
      * Returns a clock mark on the same clock that is behind this clock mark by the specified [duration].
      *
      * The returned clock mark is more _early_ when the [duration] is positive, and more _late_ when the [duration] is negative.
      */
-    operator fun minus(duration: Duration): ClockMark = plus(-duration)
+    open operator fun minus(duration: Duration): ClockMark = plus(-duration)
 }
 
 
-private class AdjustedClockMark(val mark: ClockMark, val adjustment: Duration) : ClockMark {
+private class AdjustedClockMark(val mark: ClockMark, val adjustment: Duration) : ClockMark() {
     override fun elapsed(): Duration = mark.elapsed() - adjustment
 
     override fun plus(duration: Duration): ClockMark = AdjustedClockMark(mark, adjustment + duration)

libraries/stdlib/src/kotlin/time/Clocks.kt

@@ -18,7 +18,7 @@ public expect object MonoClock : Clock
 public abstract class AbstractLongClock(protected val unit: DurationUnit) : Clock {
     protected abstract fun read(): Long
 
-    override fun mark(): ClockMark = object : ClockMark {
+    override fun mark(): ClockMark = object : ClockMark() {
         val startedAt = read()
         override fun elapsed(): Duration = (read() - startedAt).toDuration(this@AbstractLongClock.unit)
     }
@@ -30,7 +30,7 @@ public abstract class AbstractLongClock(protected val unit: DurationUnit) : Cloc
 public abstract class AbstractDoubleClock(protected val unit: DurationUnit) : Clock {
     protected abstract fun read(): Double
 
-    override fun mark(): ClockMark = object : ClockMark {
+    override fun mark(): ClockMark = object : ClockMark() {
         val startedAt = read()
         override fun elapsed(): Duration = (read() - startedAt).toDuration(this@AbstractDoubleClock.unit)
     }