theorem Th83:
  for p,q be MC-formula holds
  |-_IPC p & |-_IPC q implies  |-_IPC p '&' q
proof
  let p,q be MC-formula;
  assume
A1: |-_IPC p & |-_IPC q;
  |-_IPC p => (q => p '&' q) by Th32; then
  |-_IPC q => p '&' q by A1,Th37;
  hence thesis by A1,Th37;
end;
