reserve x,y,z for object, X,Y for set,
  i,k,n for Nat,
  p,q,r,s for FinSequence,
  w for FinSequence of NAT,
  f for Function;

theorem Th31:
  <*x,y*> is Tree-yielding iff x is Tree & y is Tree
proof
A1: x is Tree & y is Tree iff {x,y} is constituted-Trees by Th15;
  rng <*x,y*> = {x,y} by FINSEQ_2:127;
  hence thesis by A1;
end;
