In addition to the usual C++ literals, Prop introduces two new type
of literals: quark literals and regular expression literals.
Quark literals are of type Quark and are written as strings
prefixed by the # symbol.
Quark ::= #String
Quarks act like atoms in Lisp, in which name equality implies
pointer equality. In contrast, in C++ two string literals that are
identical in value do not necessarily reside in the same address.
Given strings s1 and s2, strcmp(s1,s2) == 0 does
not imply s1 == s2. Quarks are defined in the library include file
<AD/strings/quark.h>.
Regular expression literals are similar to string literals except
that they are quoted by slashes /. We'll discuss
the regular expression in section 3.1.1.