\documentclass[convert={size=250}]{standalone}

\usepackage{tikz}
\usepackage{tikz-qtree}
\usepackage{amsmath}

\begin{document}

% $(\lambda x.M)N$
% $y=f(x)$
% $\mathcal{M} \models F$

\begin{equation}
    A \models B \text{ iff } \models A \implies B
\end{equation}

\end{document}

