reserve S for non empty non void ManySortedSign;
reserve X for non-empty ManySortedSet of S;
reserve x,y,z for set, i,j for Nat;

theorem Th10:
  for t being Term of S,X
  for x st x in Subtrees t holds x is Term of S,X
  proof
    let t be Term of S,X;
    let x; assume x in Subtrees t;
    then consider p being Element of dom t such that
A1: x = t|p;
    thus x is Term of S,X by A1;
  end;
