theorem Th61:
  Free (p '&' q) = Free p \/ Free q
proof
A1: the_right_argument_of (p '&' q) = q by Th4;
  p '&' q is conjunctive & the_left_argument_of (p '&' q) = p by Th4;
  hence thesis by A1,ZF_MODEL:1;
end;
