/usr/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' generated (on backtracking), a list of possible break-points.

Note that in addition to setting the break-point, 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.