Function eval_map_transform 

pub fn eval_map_transform(
    spec: &MapSpec,
    name: &str,
    value: &Value,
    args: &[Value],
) -> Result<Value>

evaluate a single named transform from a map spec against a json value, consulting the spec’s user transforms first, then the built-ins. backs alembic map transform, the iteration loop for writing transforms: one value in, the typed result out, no inventory or backend involved.