SPECIAL TOPIC:

        CONTEXTS

        A context frame in SCL is a set of state data that describes the
        current input stream.  Amongst the members of a context frame is the
        pointer to the next character to be read, the name of the input
        file, current command being executed and the line number of the
        last line read.  The SCL interpreter maintains a stack of context
        frames.

        When you first open the RapidSIM window the SCL interpreter is
        invoked.  After initializing itself, the interpreter pushes an
        interactive frame on the context stack.  This bottom-most context
        frame is never popped off the stack while the RapidSIM window is
        open.  An interactive context is one where the input comes from the
        keyboard.

        Each time a script command is executed a new context is pushed on
        the stack.  When the end of a script is reached then its context is
        popped off the stack.  The context stack remembers where it was when
        a new script is executed enabling the interpreter to pick up from
        where it left off when a script terminates.

        The depth of the context stack is shown in square brackets prepended
        to the prompt.  This part of the prompt is absent if the stack depth
        is zero, meaning you are executing in the first interactive context.

        Commands that manipulate the context stack:

        PAUSE

        The PAUSE statement interrupts the current context (interactive or
        otherwise).

        SCRIPT

        The SCRIPT command pushes a non-interactive context and begins
        executing.

        RESUME

        The RESUME statement closes the current input file and pops a
        context from the stack.  The uncovered context is `resumed',
        that is input now comes from the context now at the top of the
        stack.

        POP

        The POP statement removes one or more contexts from the context
        stack.  The uncovered context begins executing.

SEE ALSO:

        PAUSE
        SCRIPT
        RESUME
        POP

