class SyntaxTree::BlockVar

def arg0?

automatically expanded if the values being yielded into it are an array.
When a single required parameter is declared for a block, it gets

def arg0?
  params.requireds.length == 1 && params.optionals.empty? &&
    params.rest.nil? && params.posts.empty? && params.keywords.empty? &&
    params.keyword_rest.nil? && params.block.nil?
end

Instance Methods