let ti be Function of X,INT; :: thesis: ( ti = t1 + t2 iff for s being Element of X holds ti . s = (t1 . s) + (t2 . s) )
A7: dom t1 = X by FUNCT_2:def 1;
A8: dom t2 = X by FUNCT_2:def 1;
A9: dom (t1 + t2) = (dom t1) /\ (dom t2) by VALUED_1:def 1;
hence ( ti = t1 + t2 implies for s being Element of X holds ti . s = (t1 . s) + (t2 . s) ) by A7, A8, VALUED_1:def 1; :: thesis: ( ( for s being Element of X holds ti . s = (t1 . s) + (t2 . s) ) implies ti = t1 + t2 )
A10: dom ti = X by FUNCT_2:def 1;
assume for s being Element of X holds ti . s = (t1 . s) + (t2 . s) ; :: thesis: ti = t1 + t2
then for s being set st s in X holds
ti . s = (t1 . s) + (t2 . s) ;
hence ti = t1 + t2 by A9, A7, A8, A10, VALUED_1:def 1; :: thesis: verum