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I found a counter-example and an error in my proof of this (erroneous) theorem in Funcoids and Reloids article:

Let $f\in\mathsf{FCD}$. If $R$ is a set of co-complete funcoids then $f \circ \bigcup {\nobreak}^{\mathsf{FCD}} R = \bigcup {\nobreak}^{\mathsf{FCD}} \left\langle f \circ \right\rangle R$.

A counter-example: Let $\Delta = \{ (-\epsilon;\epsilon) | \epsilon\in\mathbb{R}, \epsilon>0 \}$. Let $f = \Delta \times^{\mathsf{FCD}} \mathbb{R}$ and $R = \{ \mathbb{R}\times(\epsilon;+\infty) | \epsilon\in\mathbb{R}, \epsilon>0 \}$.

Then $\bigcup {\nobreak}^{\mathsf{FCD}} R = \bigcup R = \mathbb{R} \times (0;+\infty)$; $f \circ \bigcup {\nobreak}^{\mathsf{FCD}} R = \mathbb{R} \times \mathbb{R}$; $\bigcup {\nobreak}^{\mathsf{FCD}} \langle f \circ \rangle R = \bigcup {\nobreak}^{\mathsf{FCD}} \{ \emptyset \} = \emptyset$.

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