[Tex/LaTex] How to give a label to nodes which already contain a tikzpicture with nodes.

floatsgraphslabelstikz-pgf

I am a beginner at TikZ and LaTeX.
This is what I currently have:
enter image description here

I would like to label each of these rectangles (i.e. graphs) with a single letter, i.e. G.

This is the code for a single graph in this picture:

  \node[outer] (L) {
  \begin{tikzpicture}
    \node [inner,label=below:1] (a) at (0,0) {1};
    \node [inner] (ai) at (1,0) {1};
    \node [inner,label=below:2] (aii) at (2,0) {1};
    \draw[->] (a) edge (ai);
    \draw[->] (ai) edge (aii);
  \end{tikzpicture}
  };

Specifically, this is the code for the top leftmost graph.

I would like to give it a label L. I read online that to give a caption (label) to a tikzpicture you have to put it inside the tikz figure. I've done it for the big graph, hence you can see "Figure 3: A double pushout." below the whole picture.

But when I put the contains of this node L inside a tikzfigure, i.e.

  \node[outer] (L) {
\begin{figure}
\begin{tikzpicture}
  \node [inner,label=below:1] (a) at (0,0) {1};
  \node [inner] (ai) at (1,0) {1};
  \node [inner,label=below:2] (aii) at (2,0) {1};
  \draw[->] (a) edge (ai);
  \draw[->] (ai) edge (aii);
\end{tikzpicture}
\end{figure}
   };

It throws an error. 

! LaTeX Error: Not in outer par mode.

Also, it's really a lot "figures", which seems unnecessary for me?

I also tried this solution (which works for small nodes), so instead 

  \node[outer] (L) {

I wrote

  \node[outer, label=above:L] (L) {

But it throws this error:

! Undefined control sequence.
<argument> pgf@sh@ns@\tikzlastnode 
l.15 \draw[->] (a) edge (ai)

My code may seem silly to you, experts. So feel free to point out any mistakes! I have a feeling I'm too "wordy" for what I'm trying to achieve.

For completeness, I will say that the big tikzpicture has the following settings:

\begin{figure}[htb!]
\centering
\begin{tikzpicture}[remember picture,
inner/.style={circle,draw,inner sep=4},
outer/.style={draw,inner sep=4, outer sep=2} % deleted thick here after draw

Thanks for help everyone!

Best Answer

Another alternative, moving the draw out of {external label}.

enter image description here

Code

\documentclass[border=10pt]{standalone}

\usepackage[demo]{graphicx}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning}

\begin{document}
%
\begin{tikzpicture}[remember picture,
inner/.style={circle,draw,inner sep=4},
outer/.style={draw,inner sep=4, outer sep=2}]

 \node[outer,label=above:L]  (L) {
  \begin{tikzpicture}
    \node [inner,label=below:1] (a) at (0,0) {1};
    \node [inner] (ai) at (1,0) {1};
    \node [inner,label=below:2] (aii) at (2,0) {1};
  \end{tikzpicture}
  };
  \draw[->] (a) edge (ai)  (ai) edge (aii);


\begin{scope}[xshift=4cm]
 \node[outer,label=above:M]  (M) {
  \begin{tikzpicture}
    \node [inner,label=below:1] (a) at (0,0) {1};
%    \node [inner] (ai) at (1,0) {1};
    \node [inner,label=below:2] (aii) at (2,0) {1};
  \end{tikzpicture}
  };
  \draw[->] (a) edge  (aii);
\end{scope}

\draw[->] (L) -- (M);

\end{tikzpicture}
\end{document}
Related Question