theorem Th2: D1 c= D2 & (D2 is isotone or D1 is isotone) & x is
(m,X,D1)-derivable implies x is (m,X,D2)-derivable
proof
set f1=(m,D1)-derivables, f2=(m,D2)-derivables;
assume D1 c= D2 & (D2 is isotone or D1 is isotone); then
A1: f1.X c= f2.X by Lm16; assume x is (m,X,D1)-derivable; then
x in f1.X;
hence thesis by  A1;
end;
