theorem
  {F1,F2,F3,F4,F5} |-_IPC G implies {F2,F3,F4,F5} |-_IPC F1 => G
proof
  {F1,F2,F3,F4,F5} = {F2,F3,F4,F5} \/ {F1} by ENUMSET1:7;
  hence thesis by Th53;
end;
