theorem
  a<>0.F implies for b,c ex x st Tern(x,a,b) = c
proof
  assume
A1: a <> 0.F;
  let b,c;
  consider x,z such that
A2: Tern(x,a,z) = c & Tern(x,0.F,z) = b by A1,Def5;
  take x;
  thus thesis by A2,Def5;
end;
