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When an error is signaled,
signal searches for an active
handler for the error. A handler is a sequence of Lisp
expressions designated to be executed if an error happens in part of the
Lisp program. If the error has an applicable handler, the handler is
executed, and control resumes following the handler. The handler
executes in the environment of the
established it; all functions called within that
have already been exited, and the handler cannot return to them.
If there is no applicable handler for the error, the current command is terminated and control returns to the editor command loop, because the command loop has an implicit handler for all kinds of errors. The command loop's handler uses the error symbol and associated data to print an error message.
An error that has no explicit handler may call the Lisp debugger. The
debugger is enabled if the variable
debug-on-error (see section 18.1.1 Entering the Debugger on an Error) is non-
nil. Unlike error handlers, the debugger runs
in the environment of the error, so that you can examine values of
variables precisely as they were at the time of the error.
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