class RuboCop::Cop::Layout::EmptyLineBetweenDefs

it on each def.
doing a linear scan over siblings, so we don't want to call
efficiently access a node's predecessor; #prev_node ends up
so that we can walk over pairs of consecutive nodes and
We operate on `begin` nodes, instead of using `OnMethodDef`,

def on_begin(node)
  node.children.each_cons(2) do |prev, n|
    nodes = [prev, n]
    check_defs(nodes) if nodes.all?(&method(:def_node?))
  end
end