In the R system, missing values may have different types.
For example, if an entry is missing from a numeric vector, it is a
missing numeric value, not a missing logical value, and R
distinguishes between these two types of missing values.
The function RelevantNA returns a missing value of the same
type as the input x (as defined by typeof).
Thus, RelevantNA(3.2) returns a missing numeric value
and RelevantNA(TRUE) returns a missing logical value.
RelevantEmpty(x) returns a vector of length zero which has the
same type as x. Thus, RelevantEmpty(TRUE) is equivalent
to logical(0).
RelevantZero(x) returns a single value, of the same type as
x, that is equivalent to the number zero. For example,
RelevantZero(TRUE) returns FALSE.
The function isRelevantZero tests whether x is a single
zero value, by testing whether x is identical to
RelevantZero(x).