$T_4$-funcoids

I have added to my free ebook a definition of T_4-funcoids (generalizing T_4 topologies).

A funcoid f is T_4 iff f \circ f^{- 1} \circ f \circ f^{- 1} \sqsubseteq f \circ f^{- 1}. This can also be generalized for pointfree funcoids.

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