first to fifth return the first to fifth element
of the list x. rest returns all but the first
element of the list x. is.empty returns TRUE
iff the list x is of length 0. is.atom returns
TRUE iff the list x is of length 1.
is.composite returns TRUE iff the list x is
of length > 1. contains return TRUE iff the list
x contains an element identical to elt.
first(x)
rest(x)
second(x)
third(x)
fourth(x)
fifth(x)
is.empty(x)
is.atom(x)
is.composite(x)
contains(x, elt)