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A new invocation of a top level loop is called as a subgoal
Used to start a new invocation of the top level loop. Compiled
procedures and event handler redefinitions remain valid within the new
invocation level. Exiting with a Control-D (or end_of_file) or
calling exit_block(end) will return the control to the previous level.
go:- writeln("**** Enter prolog goals and press ^D"),
writeln("**** Execution continues...").
user compiled 144 bytes in 0.02 seconds
**** Enter prolog goals and press ^D
Entering break level 1
X = "Wed Aug 9 12:21:48 1989\n"
Leaving break level 1
**** Execution continues...
toplevel / 0, exit_block / 1