Provides W3C XML Schema as a RELAX NG datatypes library.
NAME is a symbol giving the local name of the datatype. PARAMS is a
list of pairs (PARAM-NAME . PARAM-VALUE) where PARAM-NAME is a symbol
giving the name of the parameter and PARAM-VALUE is a string giving
its value. If NAME or PARAMS are invalid, it calls rng-dt-error
passing it arguments in the same style as format; the value from
rng-dt-error will be returned. Otherwise, it returns a list. The
first member of the list is t if any string is a legal value for the
datatype and nil otherwise. The second argument is a symbol; this
symbol will be called as a function passing it a string followed by
the remaining members of the list. The function must return an object
representing the value of the datatype that was represented by the
string, or nil if the string is not a representation of any value.
The object returned can be any convenient non-nil value, provided
that, if two strings represent the same value, the returned objects must be equal. (fn NAME PARAMS)