A bitwhich object represents a boolean filter like a bit object (NAs are not allowed)
but uses a sparse representation suitable for very skewed (asymmetric) selections.
Three extreme cases are represented with logical values, no length via logical(),
all TRUE with TRUE and all FALSE with FALSE. All other selections are represented with
positive or negative integers, whatever is shorter.
This needs less RAM compared to logical (and often less than bit or which).
Logical operations are fast if the selection is asymetric (only few or almost all selected).