theorem Th41:
  Ex(x,p 'or' q) <=> (Ex(x,p) 'or' Ex(x,q)) is valid
proof
  Ex(x,p 'or' q) => (Ex(x,p) 'or' Ex(x,q)) is valid & (Ex(x,p) 'or' Ex(x,q
  )) => Ex(x,p 'or' q) is valid by Th40;
  hence thesis by Lm14;
end;
