Wow...if I'm getting this right, the "transform" simply validates each entry? In that case perhaps simply write:
```
for (String cidrBlock : cidrBlocks) {
  checkArgument(isCidrFormat(cirdBlock), "%s is not a valid CIDR", cirdBlock);
}
Iterables.addAll(this.cidrBlocks, cidrBlocks);
return this;
```
? Intent seems much clearer here to me...