amsart does not work in documentclass{combine}

When I use documentclass{combine}, $mathbb{}$ does not work. I use amsart in the imported file,
and when I pdflatex the file, it said that it is a undefined control sequence.

How can I fix it?

This the main file:

documentclass[book]{combine}
usepackage[english]{babel}
usepackage{blindtext}
usepackage{filecontents}
usepackage{etoolbox}
newcounter{savechapter}
AtBeginEnvironment{papers}{setcounter{savechapter}{value{chapter}}}
makeatletter
letoldimportimport
renewcommand*{import}{renewcommand*thesection{arabic{savechapter}.arabic{section}}oldimport}
makeatother

% The input articles


% Define abstract to be used in book class (or use report)
% http://tex.stackexchange.com/questions/51483/how-to-write-abstract-and-acknowledgement-in-book-format
makeatletter
if@titlepage
  newenvironment{abstract}{%
      titlepage
      nullvfil
      @beginparpenalty@lowpenalty
      begin{center}%
        bfseries abstractname
        @endparpenalty@M
      end{center}}%
     {parvfilnullendtitlepage}
else
  newenvironment{abstract}{%
      if@twocolumn
        section*{abstractname}%
      else
        small
        begin{center}%
          {bfseries abstractnamevspace{-.5em}vspace{z@}}%
        end{center}%
        quotation
      fi}
      {if@twocolumnelseendquotationfi}
fi
makeatother


% The main document
begin{document}
title{The collection}
author{A. N. Editor}
date{today}
maketitle

begin{papers}
    coltoctitle{Third article}
    coltocauthor{Author of third article}
    import{input}    
end{papers}

end{document}

This is the input.tex:

makeatother
documentclass[12pt,reqno]{amsart}
title{bf 12321}
author{ssss}
date{}
begin{document}
maketitle

section{Introduction}
Let $(Sigma,g_0)$ be a Riemann surface. Define $mathbb{K}$...
end{document}