     <label class="chip" title="Addition">
              <input type="checkbox" id="op-add" checked />
              <span>+</span>
            </label>
            <label class="chip" title="Subtraction">
              <input type="checkbox" id="op-sub" checked />
              <span>&minus;</span>
            </label>
            <label class="chip" title="Multiplication">
              <input type="checkbox" id="op-mul" checked />
              <span>&times;</span>
            </label>
            <label