documentclass{article}
usepackage{amsmath}
usepackage{framed}
begin{document}

begin{figure}
  begin{framed}
    begin{center}
      begin{tabular}{lr@{:space}l}
        textbf{in} & $a,b$ & textbf{integer where } $a>0$, $b>0$\
        textbf{local} & $y_1,y_2$ & textbf{integer where } $y_1=a$, $y_2=b$\
        textbf{out} & $g$ & textbf{integer}
      end{tabular}
    end{center}
    begin{equation*}
       left[begin{array}{l}
           ell_0: left[ begin{array}{l}
                 ell_1: textbf{while $y_1ne y_2$ do}\
                    qquadell_2
                    left[ begin{array}{l}
                           {ell_3:textbf{ await }y_2>y_1;ell_4: y_2=y_1-y_2}\
                           quadtextbf{or}\
                          {ell_5:textbf{ await }y_2>y_1;ell_6: y_2=y_2-y_1}
                           end{array}
                     right]\
                 ell_7: g:=y_1\
              end{array}
       right]\
       ell_8:
       end{array}
       right]
    end{equation*}
  caption{A fully labeled program GCD-F.}
  end{framed}
end{figure}

end{document}