
theorem Th9:
  for X, x being set, F being Dependency-set of X holds x in F
  implies ex a, b being Subset of X st x = [a,b]
proof
  let X, x be set, M be Dependency-set of X;
  assume
A1: x in M;
  then consider a, b being object such that
A2: [a, b] = x by RELAT_1:def 1;
  reconsider a, b as Subset of X by A1,A2,ZFMISC_1:87;
  take a,b;
  thus thesis by A2;
end;
