more comments to explain type Expression
more comments to explain type Expression

replay/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,raintank/metrictank,replay/metrictank,raintank/raintank-metric,raintank/raintank-metric,raintank/metrictank,replay/metrictank