An inference record naming a parent absent from the unit.
In a derivation, inference(rule, [], [a, b]) records that the formula was
derived from a and b. An absent parent renders the derivation uncheckable,
indicating either a dropped statement or an incorrect name.
A warning, reported only for names occurring in formula-name position. A
<source> is a <dag_source>, an <internal_source>, an <external_source>,
the literal unknown, or a bracketed list of sources, and <name> is reachable
through more than one of these: the rule of an <inference_record> and the file
name of a <file_source> are not formula names and are not resolved as such.
Neither is the literal unknown, which the grammar reads as a <name> — the
first of Tptp.Bnf.Generator's departures — and which is therefore never
reported as a missing parent. The rule also declines unless the unit names at
least one formula, so it reports nothing for a problem containing no derivation.
Before v9.3.1.2 a <source> was a <general_term>, which admitted anything at
all in the position this rule resolves.