Iterate through object keys

[dandv]

(Apologies if this is the wrong repo; landed here from https://github.com/SparkPost/sparkpost-api-documentation/blob/master/services/substitutions-reference.md)

Is there a way to iterate through object keys in the template language? For example, I want to output all keys in an object that have a non-zero value.

Thanks!