Doesn't compile: IsChunk() is not found in ThingDef

[hopeseekr]

ManagerTab_Mining.cs(57, 31): [CS1061] 'ThingDef' does not contain a definition for 'IsChunk' and no accessible extension method 'IsChunk' accepting a first argument of type 'ThingDef' could be found (are you missing a using directive or an assembly reference?)

            if ( resource.IsChunk() )