Fuente TeX:

\displaystyle f\left( \varphi \psi \to \right):=\left( f\left( \varphi \right)\to f\left( \psi \right) \right)={{f}_{3}}\left( x,\varphi ,\psi ,f\left( \varphi \right),f\left( \psi \right) \right)