* Breakpoints have been reimplemented to use the condition system. A

* Breakpoints have been reimplemented to use the condition system.  A
  breakpoint is now a condition, and the continuation of that
  condition is the continuation of the breakpoint.  (The debugger
  will not show the internal frames of the breakpoint any more.)

* The procedure BREAKPOINT now takes a condition as an additional
  optional argument.

* The environment of a breakpoint may be specified as the symbol
  CONTINUATION-ENVIRONMENT, in which case the breakpoint signaller
  will attempt to extract an environment from the continuation.  If
  that fails, it uses the REPL environment, and prints a message
  informing the user that it was unable to find an appropriate
  environment for the breakpoint.

* CMDL/START has been modified to detect the situation where the
  thread starting the CMDL is not the owner of its port.  Previously
  this check was done only for errors.  As a result of this change,
  CONDITION-TYPE:DERIVED-THREAD-ERROR has been generalized to accept
  any kind of condition, not just error conditions.

* The WRITE-RESULT output-port operation has been redefined to accept
  an additional argument: the expression that was evaluated to produce
  the value being printed.  This expression is useful if the
  CURRENT-EXPRESSION-CONTEXT operation is implemented; it is the key
  needed to get the context information.  As a result of this change,
  HOOK/REPL-WRITE was redefined to accept the same additional
  argument.

* A new procedure CURRENT-LOAD-PATHNAME has been defined.  When a file
  is being loaded, this procedure returns that file's pathname.  At
  other times, it signals an error of type CONDITION-TYPE:NOT-LOADING.
  LOAD/PUSH-HOOK! has been changed to signal this error in the same
  situation, rather than the anonymous error it signalled before.