diff --git a/cross.py b/cross.py
deleted file mode 100644
index 527a202..0000000
--- a/cross.py
+++ /dev/null
@@ -1,213 +0,0 @@
-import math
-import sys
-from kivy.graphics import Color,Line,Rectangle
-
-from point import Point
-from tools import isint_nonzero,sgn
-
-class Cross():
-    # size of central square
-    size=50
-
-    def __init__(self,x,y,**kwargs):
-        self.pos=Point(x,y)
-        self.color=kwargs.get("color",(0,0,1))
-        self.selected=False
-
-    # set position
-    def setpos(self,x,y):
-        self.pos.x=x
-        self.pos.y=y
-
-    def draw(self,**kwargs):
-        # set color
-        if not self.selected:
-            Color(*self.color)
-        else:
-            (r,g,b)=self.color
-            # darken selected
-            Color(r/2,g/2,b/2)
-
-        Rectangle(pos=((self.pos.x-1.5)*self.size,(self.pos.y-0.5)*self.size),size=(3*self.size,self.size))
-        Rectangle(pos=((self.pos.x-0.5)*self.size,(self.pos.y-1.5)*self.size),size=(self.size,3*self.size))
-
-        # stroke
-        Color(1,1,1)
-        Line(points=(
-        *((self.pos.x-0.5)*self.size,(self.pos.y-0.5)*self.size),
-        *((self.pos.x-0.5)*self.size,(self.pos.y-1.5)*self.size),
-        *((self.pos.x+0.5)*self.size,(self.pos.y-1.5)*self.size),
-        *((self.pos.x+0.5)*self.size,(self.pos.y-0.5)*self.size),
-        *((self.pos.x+1.5)*self.size,(self.pos.y-0.5)*self.size),
-        *((self.pos.x+1.5)*self.size,(self.pos.y+0.5)*self.size),
-        *((self.pos.x+0.5)*self.size,(self.pos.y+0.5)*self.size),
-        *((self.pos.x+0.5)*self.size,(self.pos.y+1.5)*self.size),
-        *((self.pos.x-0.5)*self.size,(self.pos.y+1.5)*self.size),
-        *((self.pos.x-0.5)*self.size,(self.pos.y+0.5)*self.size),
-        *((self.pos.x-1.5)*self.size,(self.pos.y+0.5)*self.size),
-        *((self.pos.x-1.5)*self.size,(self.pos.y-0.5)*self.size),
-        *((self.pos.x-0.5)*self.size,(self.pos.y-0.5)*self.size),
-        ))
-
-    # check whether a cross at pos interacts with cross
-    def check_interaction(self,pos):
-        # allow for error
-        return int(pos.x-self.pos.x+sgn(pos.x-self.pos.x)*1e-11)**2+int(pos.y-self.pos.y+sgn(pos.y-self.pos.y)*1e-11)**2>=5
-
-    # check whether a cross at position pos is touching self
-    def check_touch(self,pos):
-        rel=pos-self.pos
-        for i in [-3,-2,-1,1,2,3]:
-            # allow for error
-            if abs(rel.x-i)<1e-11 and abs(rel.y)<=4-abs(i)+1e-11 and abs(rel.y)>=3-abs(i)-1e-11:
-                    return True
-            if abs(rel.y-i)<1e-11 and abs(rel.x)<=4-abs(i)+1e-11 and abs(rel.x)>=3-abs(i)-1e-11:
-                    return True
-        return False
-
-    # find position along a line that comes in contact with the line going through pos in direction v
-    def move_on_line_to_stick(self,pos,v):
-        # relative to cross
-        return self.move_on_line_to_stick_relative(pos-self.pos,v)+self.pos
-    def move_on_line_to_stick_relative(self,x,v):
-
-        # if x is in the right quadrant
-        if abs(x.y)<=x.x:
-            # find all stuck positions on lines
-            stuck=[]
-
-            # check intersections with vertical lines
-            if v.x!=0:
-                for i in range(1,4):
-                    # candidate
-                    y=Point(i,x.y+(i-x.x)*v.y/v.x)
-                    # check that it is in the right range
-                    if abs(y.y)<=4-i and abs(y.y)>=3-i:
-                        stuck.append(y)
-            # check intersections with horizontal lines
-            if v.y!=0:
-                for i in [-3,-2,-1,1,2,3]:
-                    # candidate
-                    y=Point(x.x+(i-x.y)*v.x/v.y,i)
-                    # check that it is in the right range
-                    if y.x<=4-abs(i) and y.x>=3-abs(i):
-                        stuck.append(y)
-
-            return x.closest(stuck)
-
-        # reflect other quadrants to the right one
-        # top quadrant
-        elif abs(x.x)<=x.y:
-            closest=self.move_on_line_to_stick_relative(Point(x.y,x.x),Point(v.y,v.x))
-            return Point(closest.y,closest.x)
-        # bottom quadrant
-        elif abs(x.x)<=-x.y:
-            closest=self.move_on_line_to_stick_relative(Point(-x.y,x.x),Point(-v.y,v.x))
-            return Point(closest.y,-closest.x)
-        # left quadrant
-        else:
-            closest=self.move_on_line_to_stick_relative(Point(-x.x,x.y),Point(-v.x,v.y))
-            return Point(-closest.x,closest.y)
-
-    # move along edge of cross
-    def move_along(self,newpos,pos):
-        rel=pos-self.pos
-        # check if the particle is stuck in the x direction
-        if isint_nonzero(rel.x):
-            # check y direction
-            if isint_nonzero(rel.y):
-                # in corner
-                # two types of corners: |x|_1=3 or |x|_1=4
-                if abs(rel.x)+abs(rel.y)<3.5:
-                    if sgn(newpos.y-pos.y)==sgn(rel.y):
-                        # stuck in x direction
-                        return self.move_stuck_x(newpos,pos)
-                    elif sgn(newpos.x-pos.x)==sgn(rel.x):
-                        # stuck in y direction
-                        return self.move_stuck_y(newpos,pos)
-                else:
-                    if sgn(newpos.y-pos.y)==-sgn(rel.y):
-                        # stuck in x direction
-                        return self.move_stuck_x(newpos,pos)
-                    elif sgn(newpos.x-pos.x)==-sgn(rel.x):
-                        # stuck in y direction
-                        return self.move_stuck_y(newpos,pos)
-                # stuck in both directions
-                return pos
-            else:
-                # stuck in x direction
-                return self.move_stuck_x(newpos,pos)
-        elif isint_nonzero(rel.y):
-            # stuck in y direction
-            return self.move_stuck_y(newpos,pos)
-        # this should never happen
-        else:
-            print("error: stuck particle has non-integer relative position: (",rel.x,",",rel.y,")",file=sys.stderr)
-            exit(-1)
-    # move when stuck in the x direction
-    def move_stuck_x(self,newpos,pos):
-        # only move in y direction
-        candidate=Point(pos.x,newpos.y)
-        # do not move past corners
-        rel=pos.y-self.pos.y
-        newrel=newpos.y-self.pos.y
-        if newpos.y>pos.y:
-            if self.check_interaction(candidate)==False or (rel<math.ceil(rel)-1e-11 and newrel>math.ceil(rel)+1e-11 and math.ceil(rel)!=0):
-                # in open corner
-                if rel>math.ceil(rel)-1e-11:
-                    candidate.y=math.ceil(rel)+1+self.pos.y
-                else:
-                    candidate.y=math.ceil(rel)+self.pos.y
-        else:
-            if self.check_interaction(candidate)==False or (rel>math.floor(rel)+1e-11 and newrel<math.floor(rel)-1e-11 and math.floor(rel)!=0):
-                # in open corner
-                if rel<math.floor(rel)+1e-11:
-                    candidate.y=math.floor(rel)-1+self.pos.y
-                else:
-                    candidate.y=math.floor(rel)+self.pos.y
-        return candidate
-    # move when stuck in the y direction
-    def move_stuck_y(self,newpos,pos):
-        # only move in x direction
-        candidate=Point(newpos.x,pos.y)
-        # do not move past corners
-        rel=pos.x-self.pos.x
-        newrel=newpos.x-self.pos.x
-        if newpos.x>pos.x:
-            if self.check_interaction(candidate)==False or (rel<math.ceil(rel)-1e-11 and newrel>math.ceil(rel)+1e-11 and math.ceil(rel)!=0):
-                # in open corner
-                if rel>math.ceil(rel)-1e-11:
-                    candidate.x=math.ceil(rel)+1+self.pos.x
-                else:
-                    candidate.x=math.ceil(rel)+self.pos.x
-        else:
-            if self.check_interaction(candidate)==False or (rel>math.floor(rel)+1e-11 and newrel<math.floor(rel)-1e-11 and math.floor(rel)!=0):
-                # in open corner
-                if rel<math.floor(rel)+1e-11:
-                    candidate.x=math.floor(rel)-1+self.pos.x
-                else:
-                    candidate.x=math.floor(rel)+self.pos.x
-        return candidate
-        
-
-
-# L_infinity distance rescalled by 3 in the x direction
-def cross_distx(x,y):
-    return max(abs(x.x-y.x)/3,abs(x.y-y.y))
-# L_infinity distance rescalled by 3 in the y direction
-def cross_disty(x,y):
-    return max(abs(x.x-y.x),abs(x.y-y.y)/3)
-
-# polar description of touching cross
-def cross_polar(t):
-    # by symmetry, put angle in interval (-pi/4,pi/4), and take absolute value
-    tt=abs((t+math.pi/4)%(math.pi/2)-math.pi/4)
-    if tt<math.atan(1/3):
-        return 3/math.cos(tt)
-    elif tt<math.atan(1/2):
-        return 1/math.sin(tt)
-    else:
-        return 2/math.cos(tt)
-
-
-
diff --git a/point.py b/point.py
index 60ef0f3..eaf8dd6 100644
--- a/point.py
+++ b/point.py
@@ -43,3 +43,7 @@ class Point:
                 dist=(self-point)**2
         return closest
 
+
+# L infinity norm
+def l_infinity(x):
+    return max(abs(x.x),abs(x.y))
diff --git a/polyomino.py b/polyomino.py
new file mode 100644
index 0000000..6a52b0f
--- /dev/null
+++ b/polyomino.py
@@ -0,0 +1,128 @@
+import math
+import sys
+from kivy.graphics import Color,Line,Rectangle
+
+from point import Point,l_infinity
+from tools import isint_nonzero,sgn,in_interval
+
+class Square():
+    # size
+    size=50
+
+    def __init__(self,x,y,**kwargs):
+        self.pos=Point(x,y)
+        self.color=kwargs.get("color",(0,0,1))
+        self.selected=False
+
+    # set position
+    def setpos(self,x,y):
+        self.pos.x=x
+        self.pos.y=y
+
+    def draw(self,**kwargs):
+        # set color
+        if not self.selected:
+            Color(*self.color)
+        else:
+            (r,g,b)=self.color
+            # darken selected
+            Color(r/2,g/2,b/2)
+
+        Rectangle(pos=((self.pos.x-0.5)*self.size,(self.pos.y-0.5)*self.size),size=(self.size,self.size))
+
+    # check whether a square at pos interacts with square
+    def check_interaction(self,pos):
+        return l_infinity(pos-self.pos)>=1
+
+    # check whether a square at position pos is touching self
+    def check_touch(self,pos):
+        # allow for error
+        if in_interval(l_infinity(pos-solf.pos),1-1e-11,1+1e-11):
+                return True
+        return False
+
+    # find position along a line that comes in contact with the line going through pos in direction v
+    def move_on_line_to_stick(self,pos,v):
+        # compute intersections with four lines making up square
+        intersections=[\
+        Point(self.pos.x+1/2,pos.y+v.y/v.x*(self.pos.x+1/2-pos.x)),\
+        Point(self.pos.x-1/2,pos.y+v.y/v.x*(self.pos.x-1/2-pos.x)),\
+        Point(pos.x+v.x/v.y*(self.pos.y+1/2-pos.y),self.pos.y+1/2),\
+        Point(pos.x+v.x/v.y*(self.pos.y-1/2-pos.y),self.pos.y-1/2)\
+        ]
+
+        # compute closest one, on square
+        closest=None
+        dist=math.inf
+        for i in range(0,4):
+            # check that it is on square
+            if abs(intersections[i].x-self.pos.x)<=1/2+1e-11 and abs(intersections[i].y-self.pos.y)<=1/2+1e-11:
+                if (intersections[i]-pos)**2<dist:
+                    closest=intersections[i]
+                    dist=(intersections[i]-pos)**2
+
+        if closest==None:
+            print("error: cannot move particle at (",pos.x,",",pos.y,") to the boundary of (",self.pos.x,",",self.pos.y,")",file=sys.stderr)
+            exit(-1)
+
+        return closest
+
+    # move along edge of square
+    def move_along(self,newpos,pos):
+        rel=pos-self.pos
+        # check if the particle is stuck in the x direction
+        if isint_nonzero(rel.x):
+            # check y direction
+            if isint_nonzero(rel.y):
+                # in corner
+                if sgn(newpos.y-pos.y)==-sgn(rel.y):
+                    # stuck in x direction
+                    return self.move_stuck_x(newpos,pos)
+                elif sgn(newpos.x-pos.x)==-sgn(rel.x):
+                    # stuck in y direction
+                    return self.move_stuck_y(newpos,pos)
+                # stuck in both directions
+                return pos
+            else:
+                # stuck in x direction
+                return self.move_stuck_x(newpos,pos)
+        elif isint_nonzero(rel.y):
+            # stuck in y direction
+            return self.move_stuck_y(newpos,pos)
+        # this should never happen
+        else:
+            print("error: stuck particle has non-integer relative position: (",rel.x,",",rel.y,")",file=sys.stderr)
+            exit(-1)
+    # move when stuck in the x direction
+    def move_stuck_x(self,newpos,pos):
+        # only move in y direction
+        candidate=Point(pos.x,newpos.y)
+        # do not move past corners
+        rel=pos.y-self.pos.y
+        newrel=newpos.y-self.pos.y
+        if newpos.y>pos.y:
+            if rel<math.ceil(rel)-1e-11 and newrel>math.ceil(rel)+1e-11 and math.ceil(rel)!=0:
+                # stick to corner
+                candidate.y=math.ceil(rel)+self.pos.y
+        else:
+            if rel>math.floor(rel)+1e-11 and newrel<math.floor(rel)-1e-11 and math.floor(rel)!=0:
+                # stick to corner
+                candidate.y=math.floor(rel)+self.pos.y
+        return candidate
+    # move when stuck in the y direction
+    def move_stuck_y(self,newpos,pos):
+        # onlx move in x direction
+        candidate=Point(pos.x,newpos.x)
+        # do not move past corners
+        rel=pos.x-self.pos.x
+        newrel=newpos.x-self.pos.x
+        if newpos.x>pos.x:
+            if rel<math.ceil(rel)-1e-11 and newrel>math.ceil(rel)+1e-11 and math.ceil(rel)!=0:
+                # stick to corner
+                candidate.x=math.ceil(rel)+self.pos.x
+        else:
+            if rel>math.floor(rel)+1e-11 and newrel<math.floor(rel)-1e-11 and math.floor(rel)!=0:
+                # stick to corner
+                candidate.x=math.floor(rel)+self.pos.x
+        return candidate
+
diff --git a/tools.py b/tools.py
index a9ef4e9..651da3d 100644
--- a/tools.py
+++ b/tools.py
@@ -9,3 +9,7 @@ def isint_nonzero(x):
     if abs(x)<1e-11:
         return False
     return abs(round(x)-x)<1e-11
+
+# check that a number is in an interval
+def in_interval(x,a,b):
+    return x>=a and x<=b