code: function unionEmphasis({ change, focusKey, characters, accent, text, startOffset, endOffset, markerLength }) {
    const leftEmphEdge = getLeftEmphEdge(accent, characters, startOffset);
    const rightEmphEdge = getRightEmphEdge(accent, characters, endOffset - 1, text.length);
    const leftMarker = text.substr(leftEmphEdge, markerLength);
    let rightMarkerPos = rightEmphEdge - markerLength;
    const rightMarker = text.substr(rightMarkerPos, markerLength);
    const delCount = delInternalMarkers({
      change,
      focusKey,
      characters,
      accent,
      startPos: leftEmphEdge + markerLength,
      endPos: rightEmphEdge - markerLength
    });
    rightMarkerPos -= delCount * markerLength - 1;
    if (leftMarker !== rightMarker) {
      change.removeTextByKey(focusKey, rightMarkerPos, markerLength).
        insertTextByKey(focusKey, rightMarkerPos, leftMarker);
    }
    return change;
  }
docs: Both selection edges is on emphasis, delete all internal markers
  
  @param change
  @param focusKey
  @param characters
  @param text
  @param accent
  @param startOffset
  @param endOffset
  @param markerLength