processor Stack
  @: $;

  sink push
    ..|@Stack: $;
  end push

  source peek
    $@Stack(last) !
  end peek

  source pop
    ^@Stack(last) !
  end pop

  source empty
    $@Stack::length -> #
    <=0> 1 !
    <> 0 !
  end empty
end Stack

def myStack: [1] -> Stack;

2 -> !myStack::push

'$myStack::empty; $myStack::pop;
' -> !OUT::write
'$myStack::empty; $myStack::pop;
' -> !OUT::write
'$myStack::empty;
' -> !OUT::write

3 -> !myStack::push
'$myStack::empty; $myStack::peek;
' -> !OUT::write
'$myStack::empty; $myStack::pop;
' -> !OUT::write
'$myStack::empty;' -> !OUT::write