theorem Th57:
  {F1,F2,F3,F4} |-_IPC G implies  {F2,F3,F4} |-_IPC F1 => G
proof
  {F1,F2,F3,F4} = {F2,F3,F4} \/ {F1} by ENUMSET1:4;
  hence thesis by Th53;
end;
