With the latest lambda.r, please update to this:
```
fold(x, fn, acc) %::% . : Function : . : .
fold(EMPTY, fn, acc) %as% acc

fold(x, fn, acc=0) %when% { 
  is.null(dim(x))
} %as% {
  fold(x[-1], fn, fn(x[[1]], acc))
}

fold(x, fn, acc=0) %as% { 
  fold(x[,-1,drop=FALSE], fn, fn(x[,1], acc))
}

```