fix: pushed proxies are terms (TM)

[quickbeam123]

This fixes a parsing crash for a simple

thf(sy_v_I, type,
    i : $o > $o).

% Conjectures (2)
thf(conj_0, hypothesis,
   ((i = (^[X : $o]: X)))).
thf(conj_1, conjecture,
    (((~) = (^[X : $o]: (~ ((i @ X))))))).

Reason: the (~)-bit is parsed specially not as a negation, but already as the vNOT proxy. Therefore, it must be treated as a bool term and not formula.