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Some general conventions for return values are worth mentioning:
NULL
usually indicates failure.
Any other result usually indicates success.
The Libmarpa programmer should not overly rely on the general
conventions for return values.
In particular, -2 may sometimes be ambiguous —
both a valid return value
for success, and a potential indication of hard failure.
In this case, the programmer must distinguish the two return statuses
based on the error code,
and a programmer who is relying too heavily on the general
conventions will fall into a trap.
For a the description of the return values of
marpa_g_rule_rank_set()
,
see Rank methods.