is mentioned by

Def term_vars(t) == iterate(statevar v- > [v] statevar v'- > [v] funsymbol f- > nil freevar f- > nil trace(P)- > nil x(y)- > x @ y over t)	[term_vars]
Def tappend(tr;a) == mk_trace_env(tr.trace @ [a], tr.proj)	[tappend]

In prior sections: list 1 mb list 1 mb events mb list 2

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