src/client/pps.go
func NewCrossInput(input ...*pps.Input) *pps.Input {
	return &pps.Input{
		Cross: input,
	}
}
func NewCrossInput(input ...*pps.Input) *pps.Input {
	return &pps.Input{
		Cross: input,
	}
}
// NewCrossInput returns an input which is the cross product of other inputs.
// That means that all combination of datums will be seen by the job /
// pipeline.