sort by: {("user:a", true),("user:b", true")} 
A-null
B-null
B-baz
B-biz
A-bar
A-foo
-----------------------------------
sort by: {("user:a", false),("user:b", true")} 
A-foo
A-bar
A-null
B-null
B-baz
B-biz
-----------------------------------
sort by: {("user:a", true),("user:b", false")} 
B-biz
B-baz
A-null
B-null
A-bar
A-foo
-----------------------------------
sort by: {("user:a", false),("user:b", false")} 
A-foo
A-bar
B-biz
B-baz
A-null
B-null
-----------------------------------
sort by: {("user:b", true),("user:a", true")} 
A-null
B-null
A-bar
A-foo
B-baz
B-biz
-----------------------------------
sort by: {("user:b", false),("user:a", true")} 
B-biz
B-baz
A-null
B-null
A-bar
A-foo
-----------------------------------
sort by: {("user:b", true),("user:a", false")} 
A-foo
A-bar
A-null
B-null
B-baz
B-biz
-----------------------------------
sort by: {("user:b", false),("user:a", false")} 
B-biz
B-baz
A-foo
A-bar
A-null
B-null
-----------------------------------
no sort
B-null
A-foo
A-bar
B-baz
A-null
B-biz