source code highlighting

[ke]

These are (I believe) the rules for highlighting source code which SWI-Prolog's graphical tracer follows, and which we should probably imitate (by reusing their Prolog code):

• at call ports: highlight the goal in the body in which it occurs, if that body is visible, else highlight the first clause (which is applicable as determined by first-argument indexing?) of the called predicate
• at exit ports: highlight the end (period sign) of the successful clause
• at redo ports: highlight the end (period sign) of the previously successful clause
• at fail ports: highlight the head of the failed clause, if that clause is visible, else highlight the failed goal in the body of the clause in which it occurs (this is the inverse of the call case, so to speak)

SWI-Prolog's tracer has other, specialized ports that we need to investigate.

At the moment, we don't have clause-level highlighting, only predicate-level highlighting. This is also the reason why we don't see bound arguments at exit ports.

[xrumer]

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