Delete the dimensions of an array which have only one level.
an array (including a matrix).
x is an object with a
dim attribute (e.g., a matrix
drop returns an object like
x, but with any extents of length one removed. Any
dimnames attribute is adjusted and returned with
x: if the result is a vector the
names are taken from
dimnames (if any). If the result is a length-one vector,
the names are taken from the first dimension with a dimname.
Array subsetting (
[) performs this reduction unless used
drop = FALSE, but sometimes it is useful to invoke
drop1 which is used for dropping terms in models.