deffunc H₂( Element of NAT ) -> Element of the carrier of X = - (seq . $1);
ex seq being sequence of X st
for n being Element of NAT holds seq . n = H₂(n) from FUNCT_2:sch 4();
hence ex b₁ being sequence of X st
for n being Element of NAT holds b₁ . n = - (seq . n) ; :: thesis: verum