EGEN-TAC : TACTICEGEN-TAC is a tactic.
As defined for use NAT-DED: is a primitive tactic.
If the planned line is existentially quantified, will apply EGEN,
prompting for the term if in interactive mode.
As defined for use ETREE-NAT: is a primitive tactic.
If the planned line corresponds to a expansion node with a single
admissible expansion term, applies EGEN using that term. Same as Pfenning's
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