What's wrong?

• The only constraints on step order in XProc are those required by the connections between steps. If step “B” consumes the output of step “A” then step B can't finish before step A starts!

  <p:store href="langspec.html"/>

  <p:exec name="exec" result-is-xml="false"
          source-is-xml="false"
          command="/Users/ndw/bin/tidy"
          args="langspec.html">
    <p:input port="source"><p:empty/></p:input>
  </p:exec>