eval command

    (A) Function
        Switching an input place of a script (listening-stream) to another, 
        and restarting a command interpreter. 

    (B) Format

        eval  <script>  [ <argument> ]

    (C) Parameters
       <script>
          Set a character string. 
          This script must be one token. 

       <argument>
          Define an argument macro to be passed to the script. 

    (D) Notes
        The argument macro "$0" in a script is "eval". 
        If you want to change a listening-stream to a file,
        use the listen command. 
        A script that contains the eval command is first pushed to a stack. 
        After a new script ends, the stacked script is 
        retrieved and then resumes the suspended execution. 

    (E) Related items
        listen command
        leave command
        define command
        setenv command
        shift command