The regexp type represents a node regular expression. A node regular
expression is an object that can be used to perform an auxiliary parse of a
grove. This auxiliary parse creates a new grove that contains nodes that
group together nodes that correspond to nodes in the original grove.
The semantics of a node regular expression define for any nodelist
s and any nodelist t that is a sublist of s whether t
matches the node regular expression with respect to s. This is
defined inductively for each of the procedures that construct regexps.
s is referred to as the search list.
A nodelist s immediately precedes a nodelist t with respect
to a nodelist x that contains all the members of both s and
t if
 • the member of s that occurs latest in x occurs in x before
the element of t that occurs first in x, and 
 • there is no node in x that

 • follows in x all those members of x that occur in s, and 
 • precedes in x all those members of x that occur in t. 
(regexp? obj)
Returns #t if obj is a regexp, and otherwise returns #f.