http://purl.org/tcl/home/man/tcl8.4/TclCmd/bgerror.htm
is the official man page.

[[fill in useful tricks and examples for using bgerror]]

Does the documentation describe the calling convention?  I didn't notice
it.  My experience is that the error-handler invokes it as something like
      bgerror [firstLineOf $::errorInfo]

During development, one might rush a hack like
      proc bgerror message {
          puts stderr "An event-based script faulted with '$message'."
      }
into use.















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