◊(Local Yarn Code "Check-in [3ed11073]")

Finds all the strings from the @emph{elements} of @racket[_tx] (ignoring attributes) and
concatenates them together.

@examples[#:eval dust-eval
(tx-strs '(p [[class "intro"]] 
             (em "I’m not opening the safe") ", Wilson remembers thinking."))]

@defproc[(make-tag-predicate [sym symbol?]) (-> any/c boolean?)]
@defproc[(make-tag-predicate [sym symbol?] ...) (-> any/c boolean?)]

Returns a function (or @italic{predicate}) that returns @racket[#t] if its argument is
a @racket[_txexpr] whose tag is @racket[_sym]. This predicate is useful for passing as the
a @racket[_txexpr] whose tag matches any @racket[_sym]. This predicate is useful for passing as the
@racket[_pred] expression in functions @racket[splitf-txexpr] and @racket[findf-txexpr].

@examples[#:eval dust-eval
(define is-aside? (make-tag-predicate 'aside))
@examples[#:eval dust-eval 
(define is-aside? (make-tag-predicate 'aside 'sidebar))

(is-aside? '(q "I am not mad, Sir Topas. I say to you this house is dark."))
(is-aside? '(aside "How smart a lash that speech doth give my Conscience?"))]
(is-aside? '(aside "How smart a lash that speech doth give my Conscience?"))
(is-aside? '(sidebar "Many copies that we use today are conflated texts."))]

@defproc[(first-words [txprs (listof txexpr?)] [n exact-nonnegative-integer?]) string?]

Given a list of tagged X-expressions, returns a string containing the first @racket[_n] words found
in the string elements of @racket[_txprs], or all of the words if there are less than @racket[_n]
words available. Used by @racket[default-title].

(define (maybe-attr name attrs [missing ""])
  (define result (assoc name attrs))
  (cond
    [(pair? result) (cadr result)]
    [else missing]))

;; Returns a function will test if a txexpr's tag matches the given symbol.
(define (make-tag-predicate tagsym)
  (lambda (tx) (and (txexpr? tx) (equal? tagsym (get-tag tx)))))
(define (make-tag-predicate . tagsyms)
  (lambda (tx) (if (and (txexpr? tx) (member (get-tag tx) tagsyms)) #t #f)))

(define (tx-strs xpr)
  (cond
    [(txexpr? xpr) (apply string-append (map tx-strs (get-elements xpr)))]
    [(string? xpr) xpr]
    [else ""]))