an integer vector of length up to dim(x) (or 1,
for non-dimensioned objects). Values specify the indices to be
selected in the corresponding dimension (or along the length) of the
object. A positive value of n[i] includes the first/last
n[i] indices in that dimension, while a negative value
excludes the last/first abs(n[i]), including all remaining
indices. NA or non-specified values (when length(n) <
length(dim(x))) select all indices in that dimension. Must
contain at least one non-missing value.