The expanded CWIs:
0: kp: { x: 1, y: 1 } sparse name: '(test_foo, y)'
1: kp: { x: 1, $_path: 1 } sparse name: '(test_foo, $_path)'

The expression assigned with the CWIs by rateIndices:
y $eq 10 || First: notFirst: 0 full path: y

Test that the valid CWI assignments still remain:
y $eq 10 || First: notFirst: 0 full path: y
