theorem
  b := (f, a) in rng p or (f, a) := b in rng p implies First*NotUsed p
  <> f
proof
  assume b := (f, a) in rng p or (f, a) := b in rng p;
  then consider i being Instruction of SCM+FSA such that
A1: i in rng p and
A2: i = b := (f, a) or i = (f, a) := b;
  UsedInt*Loc i = {f} by A2,Th33;
  then
A3: {f} c= UsedI*Loc p by A1,Th35;
  not First*NotUsed p in UsedI*Loc p by Th57;
  hence thesis by A3,ZFMISC_1:31;
end;
