private long keyOf(TraceThread thread) {
		return thread == null ? -1 : thread.getKey();
	}
Returns the equivalent of executing count p-code operations less than this schedule

<p>
If {@code pStepCount} exceeds the p-code steps of this schedule, null is returned, since we
cannot know <em>a priori</em> how many p-code steps would be required to complete the
preceding instruction step. Both p-code ticks and p-code patches counts as p-code steps.

@param pStepCount the number of p-code steps to take backward
@return the resulting schedule or null if it cannot be computed
Ghidra/Debug/Framework-TraceModeling/src/main/java/ghidra/trace/model/time/schedule/TraceSchedule.java