Rules of Consequence

We may weaken a postcondition

  If   { P }  S  { R }
  and              R   ->   Q
  then { P }  S  { Q }

R -> Q means
R is stronger than Q

We may strengthen a precondition

  If             { P }  S  { R }
  and    Q    ->   P
  then { Q }  S  { R }

Therefore, we may weaken a proposition that is already proven.