Here the "Editor" is the name of the built-in field commonly known as "Modified By". However, when I try to add the content type containing the field defined above to a Document Library, I get the following exception:
"The formula refers to a column that does not exist. Check the formula for spelling mistakes or change the non-existing column to an existing column.<nativehr>0x81020057</nativehr><nativestack></nativestack>"
So, clearly the "Editor" field is not recognized as the valid field, even though it clearly is valid and exists. I even dug into a 14-hive and made sure that it exists.