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 node-list
s and any node-list 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 node-list s immediately precedes a node-list t with respect
to a node-list x that contains all the members of both s and
|• 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.|
Returns #t if obj is a regexp, and otherwise returns #f.