title: What’s the idiomatic way to guarantee some unknown value is inside a sequence?
body: <p>What’s the idiomatic way to guarantee some unknown value is inside a sequence, where that value could or could not already be in a sequence? <code>(into [] val)</code> fails if val isn't already in a seq <code>(conj [] val)</code> always works but might nest, i.e. <code>[[val...]]</code> (oh and not be nested, if already in a seq)* is it simply <code>(flatten (conj</code>?</p>