Add ability to specify number of spaces used for indentation.

package.lisp

@@ -47,6 +47,7 @@
   (:export
    #:*pretty*
    #:*indent-level*
+   #:*indent-spaces*
    #:indent
    #:write-sheet-object
    #:write-sheet-part

writer.lisp

@@ -10,6 +10,8 @@
   "Directs whether to pretty-print using whitespace or not.")
 (defvar *indent-level* 0
   "Directs the current amount of spaces used to indent.")
+(defvar *indent-spaces* 4
+  "Specifies the number of spaces to use for indentation.")
 
 ;; SHEET     ::= (BLOCK*)
 ;; BLOCK     ::= (:BLOCK SELECTOR PROPERTY*)
@@ -43,7 +45,7 @@ if *PRETTY* is non-NIL.")
           (let ((*indent-level* (+ *indent-level* 2 (length type))))
             (write-sheet-object (car selector) (cdr selector) stream)))
         (format stream "{")
-        (let ((*indent-level* (+ *indent-level* 4)))
+        (let ((*indent-level* (+ *indent-level* *indent-spaces*)))
           (dolist (block blocks)
             (when *pretty* (fresh-line stream))
             (write-sheet-object (car block) (cdr block) stream)))
@@ -55,7 +57,7 @@ if *PRETTY* is non-NIL.")
         (format stream "~a" (indent))
         (write-sheet-object (car selector) (cdr selector) stream)
         (format stream "{")
-        (let ((*indent-level* (+ *indent-level* 4)))
+        (let ((*indent-level* (+ *indent-level* *indent-spaces*)))
           (dolist (inner body)
             (when *pretty* (fresh-line stream))
             (write-sheet-object (car inner) (cdr inner) stream)))