let H be LTL-formula; ( H is atomic or H is negative or H is conjunctive or H is disjunctive or H is next or H is Until or H is Release )
A1:
H is Element of LTL_WFF
by Def10;
assume A2:
( not H is atomic & not H is negative & not H is conjunctive & not H is disjunctive & not H is next & not H is Until & not H is Release )
; contradiction
then
atom. 0 <> H
by Def11;
then A3:
not atom. 0 in {H}
by TARSKI:def 1;
atom. 0 in LTL_WFF
by Def9;
then A24:
not LTL_WFF \ {H} is empty
by A3, XBOOLE_0:def 5;
for a being set st a in LTL_WFF \ {H} holds
a is FinSequence of NAT
by Def9;
then
LTL_WFF c= LTL_WFF \ {H}
by A24, A22, A19, A16, A13, A10, A7, A4, Def9;
then
H in LTL_WFF \ {H}
by A1, TARSKI:def 3;
then
not H in {H}
by XBOOLE_0:def 5;
hence
contradiction
by TARSKI:def 1; verum