# How to create a hole in a shape and see through it (drawing of a set square)

I'd like to create a drawing that exhibit a way students could construct the tangent to a circle. This implies the drawing of a set square, and I'd want so see the remaining part of the circle in the hole of the set square. Here is where I am, but I'm stuck now. Any help would be very appreciated. If it's possible, I'd prefer a KISS solution ;-) !

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=1.8]
\node[fill, circle, minimum size=1pt, inner sep=2pt, label={[black, below left]:$O$}] (O) at (0,0) {} ;
\node[fill, circle, minimum size=1pt, inner sep=2pt, label={[black, below right]:$A$}] (A) at (-60:1) {} ;
\draw[very thick] (0,0) circle (1cm) ;
\node at (-.8,.8) {$C$} ;
\draw[very thick, gray, dashed] (O) -- (A) ;
\node[shift={(30:4)}] (B) at (A.30) {} ;
\node[shift={(-150:1)}] (C) at (A.-150) {} ;
\draw[very thick, red] (B) node[above left] {$d$} -- (C) ;
\draw[very thick, fill=gray, opacity=.5] (A) -- ++(120:.8) -- ++(0:1.6) -- (A) ;
\draw[very thick, draw, fill=white, opacity=.5] (A) ++(75:.2) coordinate (D) -- ++(120:.43) -- ++(0:.86) -- (D) ;
\end{tikzpicture}
\end{document}


Argh ! I've found the answer, thanks to jake in "Highlighting part of TikZ figure".

Here is the code :

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=1.8, remember picture]
\node[fill, circle, minimum size=1pt, inner sep=2pt, label={[black, below left]:$O$}] (O) at (0,0) {} ;
\node[fill, circle, minimum size=1pt, inner sep=2pt, label={[black, below right]:$A$}] (A) at (-60:1) {} ;
\draw[very thick] (0,0) circle (1cm) ;
\node at (-.8,.8) {$C$} ;
\draw[very thick, gray, dashed, shorten >=-7mm, shorten <=-10mm] (O) -- (A) ;
\node[shift={(30:4)}] (B) at (A.30) {} ;
\node[shift={(-150:1)}] (C) at (A.-150) {} ;
\draw[very thick, red] (B) node[above left] {$d$} -- (C) ;
\end{tikzpicture}
\begin{tikzpicture}[remember picture, overlay, even odd rule, scale=1.8]
\filldraw[draw=black, fill=lightgray, opacity=1]
(A) -- ++(120:.8) -- ++(0:1.6) -- (A)
(A) ++(75:.2) coordinate (D) -- ++(120:.43) -- ++(0:.86) -- (D) ;
\end{tikzpicture}
\end{document}