class SyntaxTree::Formatter

def seplist(list, sep = nil, iter_method = :each)

separator proc with the instance of `self`.
A similar version to the super, except that it calls back into the 
def seplist(list, sep = nil, iter_method = :each)
  first = true
  list.__send__(iter_method) do |*v|
    if first
      first = false
    elsif sep
      sep.call(self)
    else
      comma_breakable
    end
    yield(*v)
  end
end