def in_chain?(other)
      return false unless other.is_a? Listable
      chain_head.equal? other.chain_head
    end
def in_chain?(other)
      return false unless other.is_a? Listable
      chain_head.equal? other.chain_head
    end
Check if this object is in a chain.

 @param  other [Object] the object to check.
 @return [true] if this object is in the same chain as the given one.
 @return [false] otherwise.