diff --git a/terranostra.lisp b/terranostra.lisp
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+#!/usr/bin/clisp
+;;;;
+;;;; Terra Nostra is a Minecraft-like survival game for the commandline.
+;;;; 
+;;;; (c) 2018 Daniel Vedder
+;;;;
+
+(defvar *world* NIL)
+
+(defstruct patch
+	(pos '(0 0)) ;position
+	(alt 0) ;altitude
+	(streams-in NIL)
+	(streams-out NIL))
+
+(defun init-matrix (size)
+	"Create a square matrix of empty patches"
+	(do ((y 0 (1+ y)) (world NIL) (row NIL NIL))
+		((= y size) world)
+		(dotimes (x size)
+			(setf row (append row (list (make-patch :pos (list x y))))))
+		(setf world (append world (list row)))))
+
+(defun coord (x y &optional (world *world*))
+	"Return the patch at the given coordinates or NIL if out of bounds"
+	(unless (or (< x 0) (< y 0) (> x (length world)) (> y (length world)))
+		(nth x (nth y world))))