How to draw arrows between colourboxes

Question

I have written a Z specification and drawn boxes around the schemas. I'm trying to work out how to draw an arrow from schema to schema.

\documentclass{article}
\usepackage{zdra}

\begin{document}

\zlemma{
\begin{schema}{BirthdayBook}
known: \power NAME \\
birthday: NAME \pfun DATE
\where
\postcon{known=\dom birthday}
\end{schema}
}

\text{
This $InitBirthdayBook$ specifies the initial state
of the birthday book system.  It does not say explicitly that
$birthday'$ is empty, but that is implicit, because its domain
is empty.}

\zlemma{
\begin{schema}{InitBirthdayBook}
BirthdayBook~'
\where
\postcon{known' = \{ \}}
\end{schema}
}

\end{document}

I've got the boxes around the schema's fine it's just the arrow i need from box to box.

The packages needed for this to compile can be found here http://www.macs.hw.ac.uk/~lb89/packages/

Answer 1

Although OP included tag tcolorbox I think its not used but in case he wants to convert the boxes to tcolorboxes it's easy to link them. overlay and remember as options do it.

\documentclass{article}
\usepackage[most]{tcolorbox}

\begin{document}

\begin{tcolorbox}[enhanced, title=first box, overlay, remember as=first]
Some text inside a box
\end{tcolorbox}

\begin{tcolorbox}[enhanced, title=second box, overlay, remember as=second]
Some text inside a box
\end{tcolorbox}

\begin{tikzpicture}[overlay,remember picture,line width=1mm,draw=red!75!black]
\draw[->] (first.west) to[bend right] (second.west);
\end{tikzpicture}

\end{document}