A cyclic quadrilateral has its sides extended and they do meet, creating the points \(E\) and \(F\).

###### This problem is part of the Advent Calendar 2015.

As shown in the diagram, \(E\) and \(F\) form a circle with two of the vertices of the quadrilateral (say \(B\) and \(D\)).

Find the angle between the lines \(EF\) and \(AC\).

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