(<span class="hljs-selector-tag">b</span>: Line) → (c: Line) → 
    parallel <span class="hljs-selector-tag">a</span> <span class="hljs-selector-tag">b</span> → !parallel <span class="hljs-selector-tag">a</span> c → !parallel <span class="hljs-selector-tag">b</span> c )
sym = Axiom ( 
    (<span class="hljs-selector-tag">a</span>: Line) → (<span class="hljs-selector-tag">b</span>: Line) → 
    parallel <span class="hljs-selector-tag">a</span> <span class="hljs-selector-tag">b</span> → parallel