class Sass::Script::Tree::MapLiteral

def _perform(environment)
  keys = Set.new
  map = Sass::Script::Value::Map.new(Hash[pairs.map do |(k, v)|
    k, v = k.perform(environment), v.perform(environment)
    if keys.include?(k)
      raise Sass::SyntaxError.new("Duplicate key #{k.inspect} in map #{to_sass}.")
    end
    keys << k
    [k, v]
  end])
  map.options = options
  map
end