\[
\newcommand{\tr}{\Rightarrow}
\newcommand{\trs}{\tr^{\!\ast}}
\newcommand{\rlnm}[1]{\mathsf{(#1)}}
\newcommand{\rred}[1]{\xrightarrow{#1}}
\newcommand{\rreds}[1]{\mathrel{\xrightarrow{#1}\!\!^*}}
\newcommand{\cl}{\mathsf{Cl}}
\newcommand{\pow}{\mathcal{P}}
\newcommand{\matches}{\mathrel{\mathsf{matches}}}
\newcommand{\kw}[1]{\mathsf{#1}}
\]
Correct Instantiation
\[\begin{prooftree}\AxiomC{$a \matches a$}\AxiomC{$b^* \matches bbb$}\LeftLabel{$\rlnm{Concat}$}\BinaryInfC{$ab^* \matches abbb$}\end{prooftree}\]
\[\begin{prooftree}\AxiomC{$a+b \matches b$}\AxiomC{$a+b \matches a$}\LeftLabel{$\rlnm{Concat}$}\BinaryInfC{$(a+b)(a+b) \matches ba$}\end{prooftree}\]
\[\begin{prooftree}\AxiomC{$abc \matches abc$}\AxiomC{$(b + \epsilon) \matches \epsilon$}\LeftLabel{$\rlnm{Concat}$}\BinaryInfC{$(abc)(b + \epsilon) \matches abcb$}\end{prooftree}\]