theorem
  a<>0.F & Tern(x,a,b) = Tern(x9,a,b) implies x=x9
proof
  assume
A1: a<>0.F & Tern(x,a,b) = Tern(x9,a,b);
  Tern(x,0.F,b) = b & Tern(x9,0.F,b) = b by Def5;
  hence thesis by A1,Def5;
end;
