code: func filterShowFromLocation(loc *Location, showFrom *regexp.Regexp) bool {
  	if m := loc.Mapping; m != nil && showFrom.MatchString(m.File) {
  		return true
  	}
  	if i := loc.lastMatchedLineIndex(showFrom); i >= 0 {
  		loc.Line = loc.Line[:i+1]
  		return true
  	}
  	return false
  }
docs: // filterShowFromLocation tests a showFrom regex against a location, removes
  // lines after the last match and returns whether a match was found. If the
  // mapping is matched, then all lines are kept.