theorem Th67:
  a is odd & 2|^n divides a*b implies 2|^n divides b
  proof
    assume
    A1: a is odd & 2|^n divides a*b; then
    2|^n,a are_coprime by NAT_5:3;
    hence thesis by A1,WSIERP_1:29;
  end;
