Nothing

```
weights.matrix <- function(match.matrix, treat) {
weights <- weights_matrix(match.matrix, treat)
if (sum(weights)==0)
stop("No units were matched.", call. = FALSE)
else if (sum(weights[treat == 1])==0)
stop("No treated units were matched.", call. = FALSE)
else if (sum(weights[treat == 0])==0)
stop("No control units were matched.", call. = FALSE)
return(setNames(weights, names(treat)))
}
```

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