The operator compares each item in the selection against the value of the parameter, retaining only those items that pass the specified condition.
For example:
```
[[2]compare:number:eq[3]] returns nothing
[[2]compare:number:lt[3]] returns "2"
[[2]compare:number:eq[2]] returns "2"
```
Note that several of the variants of the <<.op compare>> operator are synonyms for existing operators, and are provided in the interests of consistency. For example, `compare:string:eq[x]` is a synonym for `match[x]`.