Function f 

fn f<'a>(y: &'a Yaml, camel: &'static str, snake: &'static str) -> &'a Yaml

Look up one field.

proto3 JSON says a decoder must accept both the lowerCamelCase name and the original proto name, and real traffic contains both — SDKs send camel, the Collector’s otlpjson marshaler can send either. Two exact hash probes cost less than one fuzzy scan of the mapping, so both are just tried.