theorem
 for f being Function, M being ManySortedSet of I
   holds (f+*M)|I = M
proof
 let f be Function, M be ManySortedSet of I;
A1: dom(f|I) c= I by RELAT_1:58;
A2: dom M = I by PARTFUN1:def 2;
 thus (f+*M)|I = f|I +* M|I by FUNCT_4:71
   .= f|I +* M
   .= M by A1,A2,FUNCT_4:19;
end;
