theorem
  {F1,F2,F3,F4,F5,F6,F7} |-_IPC G implies
  {F2,F3,F4,F5,F6,F7} |-_IPC F1 => G
proof
  {F1,F2,F3,F4,F5,F6,F7} = {F2,F3,F4,F5,F6,F7} \/ {F1} by ENUMSET1:16;
  hence thesis by Th53;
end;
