code: function checkVertStretch (up, dn, g, doP) /* non-MathML */ {
    if (g.nodeName.toLowerCase() == 'mo' && g.childNodes.length == 1) {
      var c = g.firstChild, s = c.data;
      if (c.nodeType == 3 /* Text */ && (up > 0.9 || dn > 0.9)
        && (M.prefix_[s] < 25 || M.postfix_[s] < 25
          || '|\u2016\u221A' /* ‖ &radic; */.indexOf(s) != -1 || doP)) {
        var r = (up + dn) / 1.2, radicQ = s == '\u221A',
          v = (radicQ ? 0.26 : 0.35) + ((radicQ ? 0.15 : 0.25) - dn) / r;
        g.style.fontSize = r.toFixed(3) + 'em';
        g.style.verticalAlign = v.toFixed(3) + 'em';
        g.fmUp = up;
        g.fmDn = dn;
        g.style.display = 'inline-block';
        g.style.transform = g.style.msTransform = g.style.MozTransform =
          g.style.WebkitTransform = 'scaleX(0.5)';
      }
    }
  }
docs: fmUp is mid-x to outer top, fmDn is mid-x to outer bottom, both approx. & in parent ems