Any thing which implements this represents a Callable-like node which can have a block associated with it as part of that call. The calls which can be this are: RubyCallNode, FCallNode, VCallNode, and SuperNode. Blocks (the IterNode that this interface refers to can be either an IterNode ( {...} or do ... end ) or a BlockPassNode (&block). It is likely we can remove this interface once the parser explicitly passes all iters into the callable node during construction.