And also, the block [echo :x] doesn't use declarations of parameters - instead the ":x" is an operation that pulls in the next argument from the call site and s

And also, the block [echo :x] doesn't use declarations of parameters - instead the ":x" is an operation that pulls in the next argument from the call site and stores it into the local binding x in the closure. Almost Forth-ish, but there is no stack.