defpred S₁[ object ] means (seq_id $1) (#) (seq_id $1) is summable ;
consider IT being set such that
A1: for x being object holds
( x in IT iff ( x in the_set_of_RealSequences & S₁[x] ) ) from XBOOLE_0:sch 1();
for x being object st x in IT holds
x in the_set_of_RealSequences by A1;
then IT is Subset of the_set_of_RealSequences by TARSKI:def 3;
hence ex b₁ being Subset of Linear_Space_of_RealSequences st
for x being object holds
( x in b₁ iff ( x in the_set_of_RealSequences & (seq_id x) (#) (seq_id x) is summable ) ) by A1; :: thesis: verum