NEG-OR-ELIM-DUP-TAC : TACTICNEG-OR-ELIM-DUP-TAC is a tactic.
As defined for use ETREE-NAT: is a primitive tactic.
If planned line is FALSEHOOD, and a support line is a negated disjunction
both of whose disjuncts is essential, applies eneg rule,
adding the line with its other expansions as a support.
Same as Pfenning's tactic 219.
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