deffunc H₂( Element of REAL ) -> Element of REAL = Sum ($1 rExpSeq);
consider f being Function of REAL,REAL such that
A1: for d being Element of REAL holds f . d = H₂(d) from FUNCT_2:sch 4();
take f ; :: thesis: for d being real number holds f . d = Sum (d rExpSeq)
let d be real number ; :: thesis: f . d = Sum (d rExpSeq)
d in REAL by XREAL_0:def 1;
hence f . d = Sum (d rExpSeq) by A1; :: thesis: verum