Record unchanged_on (m_before m_after: mem) : Prop := mk_unchanged_on {
  unchanged_on_nextblock:
    Ple (nextblock m_before) (nextblock m_after);
  unchanged_on_perm:
    forall b ofs k p,
    P b ofs -> valid_block m_before b ->
    (perm m_before b ofs k p <-> perm m_after b ofs k p);
  unchanged_on_contents:
    forall b ofs,
    P b ofs -> perm m_before b ofs Cur Readable ->
    ZMap.get ofs (PMap.get b m_after.(mem_contents)) =
    ZMap.get ofs (PMap.get b m_before.(mem_contents))
}.Record unchanged_on (m_before m_after: mem) : Prop := mk_unchanged_on {
  unchanged_on_nextblock:
    Ple (nextblock m_before) (nextblock m_after);
  unchanged_on_perm:
    forall b ofs k p,
    P b ofs -> valid_block m_before b ->
    (perm m_before b ofs k p <-> perm m_after b ofs k p);
  unchanged_on_contents:
    forall b ofs,
    P b ofs -> perm m_before b ofs Cur Readable ->
    ZMap.get ofs (PMap.get b m_after.(mem_contents)) =
    ZMap.get ofs (PMap.get b m_before.(mem_contents))
}.Record unchanged_on (m_before m_after: mem) : Prop := mk_unchanged_on {
  unchanged_on_nextblock:
    Ple (nextblock m_before) (nextblock m_after);
  unchanged_on_perm:
    forall b ofs k p,
    P b ofs -> valid_block m_before b ->
    (perm m_before b ofs k p <-> perm m_after b ofs k p);
  unchanged_on_contents:
    forall b ofs,
    P b ofs -> perm m_before b ofs Cur Readable ->
    ZMap.get ofs (PMap.get b m_after.(mem_contents)) =
    ZMap.get ofs (PMap.get b m_before.(mem_contents))
}.Record unchanged_on (m_before m_after: mem) : Prop := mk_unchanged_on {
  unchanged_on_nextblock:
    Ple (nextblock m_before) (nextblock m_after);
  unchanged_on_perm:
    forall b ofs k p,
    P b ofs -> valid_block m_before b ->
    (perm m_before b ofs k p <-> perm m_after b ofs k p);
  unchanged_on_contents:
    forall b ofs,
    P b ofs -> perm m_before b ofs Cur Readable ->
    ZMap.get ofs (PMap.get b m_after.(mem_contents)) =
    ZMap.get ofs (PMap.get b m_before.(mem_contents))
}.Record unchanged_on (m_before m_after: mem) : Prop := mk_unchanged_on {
  unchanged_on_nextblock:
    Ple (nextblock m_before) (nextblock m_after);
  unchanged_on_perm:
    forall b ofs k p,
    P b ofs -> valid_block m_before b ->
    (perm m_before b ofs k p <-> perm m_after b ofs k p);
  unchanged_on_contents:
    forall b ofs,
    P b ofs -> perm m_before b ofs Cur Readable ->
    ZMap.get ofs (PMap.get b m_after.(mem_contents)) =
    ZMap.get ofs (PMap.get b m_before.(mem_contents))
}.

Lemma loadbytes_unchanged_on: forall m m' b ofs n bytes, unchanged_on m m' -> (forall i, ofs <= i < ofs + n -> P b i) -> loadbytes m b ofs n = Some bytes -> loadbytes m' b ofs n = Some bytes.