class Sass::Tree::Visitors::Perform

def visit_function(node)
  env = Sass::Environment.new(@environment, node.options)
  if node.normalized_name == 'calc' || node.normalized_name == 'element' ||
      node.name == 'expression' || node.name == 'url'
    @@function_name_deprecation.warn(node.filename, node.line, <<WARNING)
ming a function "#{node.name}" is disallowed and will be an error in future versions of Sass.
is name conflicts with an existing CSS function with special parse rules.
RNING
  end
  @environment.set_local_function(node.name,
    Sass::Callable.new(node.name, node.args, node.splat, env,
                       node.children, false, "function", :stylesheet))
  []
end