theorem
  {F1,F2,F3,F4,F5,F6} |-_IPC G implies
  {F2,F3,F4,F5,F6} |-_IPC F1 => G
proof
  {F1,F2,F3,F4,F5,F6} = {F2,F3,F4,F5,F6} \/ {F1} by ENUMSET1:11;
  hence thesis by Th53;
end;
