theorem
  {p,q} |- r iff {p '&' q} |- r
proof
  thus {p,q} |- r implies {p '&' q} |- r
  proof
    assume r in Cn({p,q});
    then r in Cn({p} \/ {q}) by ENUMSET1:1;
    hence r in Cn({p '&' q}) by Th88;
  end;
  assume r in Cn({p '&' q});
  then r in Cn({p} \/ {q}) by Th88;
  hence r in Cn({p,q}) by ENUMSET1:1;
end;
