So I'm writing this:

    ++++++ begin code

    [...]

    \theoremstyle{definition}

    \newtheorem{theo}{Theorem}
    \newenvironment{theorem}[1][]
      {\begin{theo}[#1]\begin{leftbar}}
      {\end{leftbar}\end{theo}}

    \newtheorem{lemma}[theorem]{Lemma}

    [...]

    ------ end code

    And the compiler says:

    ./pbw.tex:29: LaTeX Error: No counter 'theorem' defined.

    See the LaTeX manual or LaTeX Companion for explanation.
    Type  H <return>  for immediate help.
     ...

    l.29 \newtheorem{lemma}[theorem]{Lemma}

    ?