/usr/local/lib/swipl/library/prolog_breakpoints.pl
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 set_breakpoint(+File, +Line, +Char, -Id) is det
 set_breakpoint(+Owner, +File, +Line, +Char, -Id) is det
Put a breakpoint at the indicated source-location. File is a current sourcefile (as reported by source_file/1). Line is the 1-based line in which Char is. Char is the position of the break.

First, '$clause_from_source'/4 uses the SWI-Prolog clause-source information to find the last clause starting before Line. '$break_pc' generates (on backtracking), a list of possible breakpoints.

Note that in addition to setting the breakpoint, the system must be in debug mode for the breakpoint to take effect. With threading enabled, there are various different ways this may be done. See debug/0, tdebug/0 and tdebug/1. Therefore, this predicate does not enable debug mode.

Arguments:
Owner- denotes the file that owns the clause. set_breakpoint/5 is used to set breakpoints in an included file in the context of the Owner main file. See source_file_property/2.