deffunc H₁( Element of S1) -> set = union { (M . s1) where s1 is SortSymbol of S1 : s1 <= $1 } ;
consider f being Function such that
A1: ( dom f = the carrier of S1 & ( for d being Element of S1 holds f . d = H₁(d) ) ) from FUNCT_1:sch 4();
reconsider f = f as ManySortedSet of by A1, PARTFUN1:def 4, RELAT_1:def 18;
reconsider f1 = f as ManySortedSet of ;
f1 is order-sorted

proof

let r1, r2 be Element of S1; :: according to OSALG_1:def 18 :: thesis: ( not r1 <= r2 or f1 . r1 c= f1 . r2 )
assume A2: r1 <= r2 ; :: thesis: f1 . r1 c= f1 . r2
let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in f1 . r1 or x in f1 . r2 )
assume A3: x in f1 . r1 ; :: thesis: x in f1 . r2
x in union { (M . s2) where s2 is SortSymbol of S1 : s2 <= r1 } by A1, A3;
then consider y being set such that
A4: ( x in y & y in { (M . s2) where s2 is SortSymbol of S1 : s2 <= r1 } ) by TARSKI:def 4;
consider s3 being SortSymbol of S1 such that
A5: ( y = M . s3 & s3 <= r1 ) by A4;
s3 <= r2 by A2, A5, ORDERS_2:26;
then y in { (M . s2) where s2 is SortSymbol of S1 : s2 <= r2 } by A5;
then x in union { (M . s2) where s2 is SortSymbol of S1 : s2 <= r2 } by A4, TARSKI:def 4;
hence x in f1 . r2 by A1; :: thesis: verum

end;

then reconsider f2 = f1 as OrderSortedSet of ;
take f2 ; :: thesis: for s being SortSymbol of S1 holds f2 . s = union { (M . s1) where s1 is SortSymbol of S1 : s1 <= s }
thus for s being SortSymbol of S1 holds f2 . s = union { (M . s1) where s1 is SortSymbol of S1 : s1 <= s } by A1; :: thesis: verum