Each function searches tree for occurrences of a particular old item of an element or subexpression that satisfies the test. nsubst, nsubst-if, and nsubst-if-not are like subst, subst-if, and subst-if-not respectively, except that the original tree is modified. subst makes a copy of tree, substituting new for every subtree or leaf of tree (whether the subtree or leaf is a car or a cdr of its paren