In LaTeX, we often write { x mid x in X land Phi(x) } for a set-builder notation. In MathML, it will be like

<mo>{</mo>
<mi>x</mi>
<mo>|</mo>
<mi>x</mi>
<mo>∈</mo>
<mi>X</mi>
<mo>∧</mo>
<mi>Φ</mi>
<mo>&af;</mo>
<mo>(</mo>
<mi>x</mi>
<mo>)</mo>
<mo>}</mo>

but what symbol should I use for the vertical bar? U+007C | or U+2223 ∣?