diff --git a/common.scm b/common.scm
index 368c43b..99de2ef 100644
--- a/common.scm
+++ b/common.scm
@@ -1,3 +1,5 @@
+(import (srfi :43)) ; vector extensions
+
 ; returns a list of numbers parsed from the first line of the file,
 ; separated by commas
 (define (read-comma-separated-numbers file)
@@ -14,12 +16,16 @@
 
 (define-record-type matrix (fields width height data))
 
+; builds a matrix record from the given 2D vector
+; vectors inside data vector are expected to all be the same length
 (define (matrix-from-data data)
   (make-matrix
     (vector-length (vector-ref data 0))
     (vector-length data)
     data))
 
+; returns value at x,y coordinates in matrix
+; if coordinate is out of bounds, returns default
 (define (matrix-get matrix x y default)
   (if (or (< x 0)
 	  (< y 0)
@@ -28,6 +34,8 @@
     default
     (vector-ref (vector-ref (matrix-data matrix) y) x)))
 
+; set value at x,y in matrix with given one
+; if coordinate is out of bounds, nothing happens
 (define (matrix-set! matrix x y value)
   (when (and (>= x 0)
 	     (>= y 0)
@@ -35,6 +43,12 @@
 	     (< y (matrix-height matrix)))
     (vector-set! (vector-ref (matrix-data matrix) y) x value)))
 
+(define (matrix-print matrix)
+  (let y-loop [(y 0)]
+    (printf "~a~%" (vector-ref (matrix-data matrix) y))
+    (when (< y (- (matrix-height matrix) 1))
+      (y-loop (+ y 1)))))
+
 ; parse 0-9 numerical data from file and return a matrix
 (define (load-matrix file)
   (let y-loop [(heightmap '())]