While the Scheme process is running, the modelines of all buffers in
scheme-mode are modified to show the state of the process.  The possible
states and their meanings are:

  input		waiting for input
  run		evaluating
  gc		garbage collecting

The process buffer's modeline displays the command interpreter level and type,
where the name is normally displayed.

Scheme maintains a stack of command interpreters.  Every time an error or
breakpoint occurs, the current command interpreter is pushed on the command
interpreter stack, and a new command interpreter is started.  One example of
why this is done is so that an error that occurs while you are debugging
another error will not destroy the state of the initial error, allowing you to
return to it after the second error has been fixed.

The command interpreter level indicates how many interpreters are in the
command interpreter stack.  It is initially set to one, and it is incremented
every time that stack is pushed, and decremented every time it is popped.  The
following commands are useful for manipulating the command interpreter stack:

  C-c b		pushes the stack once
  C-c u		pops the stack once
  C-c g		pops everything off
  C-c x		aborts evaluation, doesn't affect stack

Some possible command interpreter types and their meanings are:

  [Evaluator]	read-eval-print loop for evaluating expressions
  [Debugger]	single character commands for debugging errors
  [Where]	single character commands for examining environments