Axiom parallelAxiom { parallel <span class="hljs-comment">( a, b )</span>, !parallel <span class="hljs-comment">( a, c )</span> } ⇒ !parallel <span class="hljs-comment">( b, c )</span>
Axiom sym { parallel <span class="hljs-comment">( a, b )</span> } ⇒ parallel <span class="hljs-comment">( b, a )</span>
<span class="hljs-comment">// 证明</span>
parallelogram { p: parallel <span class="hljs-comment">( a, b )</span>, 
                q: parallel <span class="hljs-comment">( c, d )</span>, 
             