ode class="computeroutput"><span class="identifier">a</span></code> and <code class="computeroutput"><span class="identifier">b</span></code> are parsers, and <code class="computeroutput"><span class="identifier">A</span></code>
        is the type of the attribute of <code class="computeroutput"><span class="identifier">a</span></code>,
        and <code class="computeroutput"><span class="identifier">B</span></code> is the type of the
        attribute of <code class="computeroutput"><span class="identifi