is_pairlist()
checks that x
has type pairlist
or NULL
.
NULL
is treated as a pairlist because it is the terminating
node of pairlists and an empty pairlist is thus the NULL
object itself.
is_node()
checks that x
has type pairlist
.
In other words, is_pairlist()
tests for the data structure while
is_node()
tests for the internal type.
is_pairlist(x)is_node(x)
Object to test.
is_lang()
tests for language nodes.