code: function getArrowIndex(read: Alignment): number {
    var i, op, ops = read.cigarOps;
    if (read.getStrand() == '-') {
      for (i = 0; i < ops.length; i++) {
        op = ops[i];
        if (op.op == 'S') continue;
        if (op.op == 'M') return i;
        return -1;
      }
    } else {
      for (i = ops.length - 1; i >= 0; i--) {
        op = ops[i];
        if (op.op == 'S') continue;
        if (op.op == 'M') return i;
        return -1;
      }
    }
    return -1;
  }
docs: Determine which alignment segment to render as an arrow. This is either the first or last 'M' section, excluding soft clipping.