canTruncate (annos, sel) {
    if (annos.length !== 1) return false
    const annoSel = annos[0].getSelection()

    return (sel.isLeftAlignedWith(annoSel) || sel.isRightAlignedWith(annoSel)) &&
           !sel.contains(annoSel) &&
           !sel.isCollapsed()
  }
Checks if annotation could be truncated.
    There should be overlap with only a single annotation,
    selection should also have boundary in common with this annotation.

    @param {Array} annos annotations
    @param {Object} sel selection

    @returns {Boolean} Whether or not annotation could be truncated.