As described in a thread in the groovy user list here:
An implementation of groupBy that receives a list or an array of closures as criteria for grouping would be useful for making nested groupings.
groupBy with one closure criterion would behave like the current implementation of groupBy.
|Field||Original Value||New Value|
|Assignee||Guillaume Laforge [ guillaume ]|
|Status||Open [ 1 ]||Resolved [ 5 ]|
|Fix Version/s||1.8.1 [ 17223 ]|
|Fix Version/s||1.9-beta-1 [ 17153 ]|
|Resolution||Fixed [ 1 ]|
|Status||Resolved [ 5 ]||Closed [ 6 ]|