class Sass::Tree::Visitors::Perform

def visit_variable(node)
  env = @environment
  env = env.global_env if node.global
  if node.guarded
    var = env.var(node.name)
    return [] if var && !var.null?
  end
  val = node.expr.perform(@environment)
  if node.expr.source_range
    val.source_range = node.expr.source_range
  else
    val.source_range = node.source_range
  end
  env.set_var(node.name, val)
  []
end