How do i check if a variable is set or not in Less like Sass's variable_exists($name)
<p>I am trying to convert Sass to Less but I got problem. In Sass code there are <a href="http://sass-lang.com/documentation/Sass/Script/Functions.html#variable_exists-instance_method" rel="nofollow"><code>variable_exists($name)</code></a> but Less does not have similar function. So I converted code below</p>

<p><strong>Sass</strong></p>

<pre><code>.test {
    background: if(variable-exists(background), $small-font-size, red);
}
</code></pre>

<p>↓</p>

<p><strong>Less</strong></p>

<pre><code>.define-test(@background: red) {
    background: @background;
}

.test {
    .define-test(); 
}
</code></pre>

<p>It works but I got problem if it will be complicated code. Please tell me how to convert complicated Sass code in Less below: </p>

<p><strong>Sass</strong></p>

<pre><code>.test {
    font: if(variable-exists(font-size), $font-size, 100%)/#{if(variable-exists(line-height), $line-height, 1.5)};
}
</code></pre>

<p>Thanks in advance.</p>