====== "cross" measure ======

===== Purpose =====

Find the time or frequency where a probed value crosses a reference.

===== Syntax =====

  cross(probe="p(d)" cross=value begin=time end=time rise fall last first)
===== Parameters =====

^arg name       ^default  ^description  ^
|probe          |required   |The name of a probe to measure.  |
|cross          |0          |The value it crosses.  |
|begin\\ after  |-infinity  |The window begins at this time or frequency.  |
|end\\ before   |+infinity  |The window ends at this time or frequency.  |
|first          |true       |Find the first crossing in the window.  (Cancels last.)  |
|last           |false      |Find the last crossing in the window.  (Cancels first.)  | 
|rise           |true       |Consider only rising crossings.  (Cancels fall.)  |
|fall           |false      |Consider only falling crossings.  (Cancels rise.)  |
===== Caveats =====

For this function to detect a crossing, it must actually cross.
  * A steady state at the cross value is not a crossing.
  * Being at the cross value and leaving is not a crossing.
  * Merely touching (becoming equal to) the cross value and leaving from the same side is not a crossing.

===== Diagnostics =====

If there is no crossing within the specified range, it returns "Inf" (infinity).

If you didn't store the waveform (see store command), it throws a "no match" exception, which prints "no match".

===== Examples =====

  measure t2 = cross(probe="v(out)" cross=0 rise last)
Find the last time that v(out) crosses 0, going up.
  measure t1 = cross(probe="v(out)" cross=0 rise last before=t2)
Find the last rising zero crossing before t2.
  param period = {t2-t1}
  param frequency = {1/period}
Params are interpreted like a functional language so they will be updated whenever t1 or t2 changes.