\version "2.12.0"

#(define counter-alist '())

#(define-markup-command (counter layout props name) (string?)
  (let* ((oldval (assoc-ref counter-alist name))
         (newval (if (number? oldval) (+ oldval 1) 1)))
  (set! counter-alist (assoc-set! counter-alist name newval))
  (interpret-markup layout props
    (markup (number->string newval)))))

#(define-markup-command (setcounter layout props name value) (string? number?)
  (set! counter-alist (assoc-set! counter-alist name (- value 1)))
  (interpret-markup layout props (make-counter-markup name)))

\markup {This is a counter: \counter #"testing" }
\markup {It is increasing: \counter #"testing" }
\markup {It is increasing: \counter #"testing" }
\markup {Setting to a specific value works, too:}
\markup {Set to 15: \setcounter #"testing" #15 }
\markup {It is increasing: \counter #"testing" }
\markup {It is increasing: \counter #"testing" }
\markup {It is increasing: \counter #"testing" }
\markup {It is increasing: \counter #"testing" }
\markup {It is increasing: \counter #"testing" }