theorem Th37:
  All(x,p '&' q) <=> (All(x,p) '&' All(x,q)) is valid
proof
  All(x,p '&' q) => (All(x,p) '&' All(x,q)) is valid & (All(x,p) '&' All(x
  ,q)) => All(x,p '&' q) is valid by Th36;
  hence thesis by Lm14;
end;
