diff --git a/src/sgfParser/__init__.py b/src/sgfParser/__init__.py
--- a/src/sgfParser/__init__.py
+++ b/src/sgfParser/__init__.py
@@ -4,8 +4,24 @@ def skipWhitespace(s,start):
 	return i
 
 
+def lineNumber(s,i):
+	k=0
+	r=0
+	for (r,line) in enumerate(s.splitlines(True)):
+		k+=len(line)
+		if k>=i: break
+	return r+1
+
+
 class ParserError(Exception):
-	def __init__(self,line,col,message):
-		self.line=line
-		self.col=col
+	def __init__(self,message,s,i):
+		# self.line=line
+		# self.col=col
+		# !! check for i<len(s)
+		print(message)
+		print(s[i:])
 		self.message=message
+
+
+class ParserWarning(ParserError):
+	pass
diff --git a/src/sgfParser/collection.py b/src/sgfParser/collection.py
--- a/src/sgfParser/collection.py
+++ b/src/sgfParser/collection.py
@@ -1,17 +1,19 @@
 from sgfParser.node import Node
-from . import skipWhitespace
+from . import skipWhitespace, ParserError
 
 
 class Collection:
 	def __init__(self,s):
 		self.gameTrees=[]
-		i,x=GameTree.create(s,0)
-		if x is None:
-			print("error when parsing Collection")
-			return
-		while x is not None:
+		i=skipWhitespace(s,0)
+		if i>=len(s): return
+		elif not GameTree.fits(s,i):
+			raise ParserError("expected a GameTree starting with '('",s,i)
+		while GameTree.fits(s,i):
+			i,x=GameTree.create(s,i)
 			self.gameTrees.append(x)
-			i,x=GameTree.create(s,i)
+		if i<len(s):
+			raise ParserError("expected EOF",s,i)
 
 	def listGames(self):
 		for tree in self.gameTrees:
@@ -24,38 +26,38 @@ class GameTree:
 		self.branches=[]
 
 	@staticmethod
+	def fits(s,i):
+		return i<len(s) and s[i]=="("
+
+	@staticmethod
 	def create(s,start):
+		assert GameTree.fits(s,start)
 		res=GameTree()
-		i=skipWhitespace(s,start)
-		if i>=len(s) or s[i]!="(":
-			# print("error when parsing GameTree")
-			return (start,None)
-		i,x=Node.create(s,i+1)
-		if x is None:
-			# print("error when parsing GameTree")
-			return (i,None)
+
+		i=skipWhitespace(s,start+1)
+		if not Node.fits(s,i):
+			raise ParserError("expected a Node starting with ';'",s,i)
+
 		y=None
-		while x is not None:
+		while Node.fits(s,i):
+			i,x=Node.create(s,i)
 			res.nodes.append(x)
 			if y: y.addChild(x)
 			x.setParent(y)
 			y=x
 			i=skipWhitespace(s,i)
-			i,x=Node.create(s, i)
-		i=skipWhitespace(s,i)
-		i,x=GameTree.create(s,i)
-		while x is not None:
+
+		while GameTree.fits(s,i):
+			i,x=GameTree.create(s,i)
 			res.branches.append(x)
 			subroot=x.getNode(0)
-			if subroot:
-				subroot.setParent(y)
+			subroot.setParent(y)
 			if y: y.addChild(subroot)
 			i=skipWhitespace(s,i)
-			i,x=GameTree.create(s,i)
-		if s[i]!=")":
-			# print("error when parsing GameTree")
-			return (i,None)
-		return (i+1,res)
+		if i>=len(s) or s[i]!=")":
+			raise ParserError("expected end of the GameTree marked by ')'",s,i)
+		i=skipWhitespace(s,i+1)
+		return (i,res)
 
 	## Expand multiple games into distinct GameTrees and yield each.
 	def listGames(self):
diff --git a/src/sgfParser/node.py b/src/sgfParser/node.py
--- a/src/sgfParser/node.py
+++ b/src/sgfParser/node.py
@@ -1,4 +1,4 @@
-from . import skipWhitespace, ParserError
+from . import skipWhitespace, ParserWarning
 from .property import Property, GAME_INFO
 
 
@@ -9,21 +9,23 @@ class Node:
 		self.children=[]
 
 	@staticmethod
+	def fits(s,i):
+		return i<len(s) and s[i]==";"
+
+	@staticmethod
 	def create(s,start):
+		assert Node.fits(s,start)
 		res=Node()
-		if s[start]!=";":
-			# print("error when parsing Node")
-			return (start,None)
+
 		i=skipWhitespace(s,start+1)
-		i,x=Property.create(s,start+1)
-		while x is not None:
+		while Property.fits(s,i):
+			i,x=Property.create(s,i)
 			if x.name in res.properties:
-				print(res.properties)
-				raise ParserError(0,0,'duplicate "{0}" property in node at position {1}. second value ignored'.format(x.name,start))
+				# !! raise or log or ignore
+				raise ParserWarning('duplicate "{0}" property in a node. second value ignored'.format(x.name),s,i)
 			else:
 				res.properties[x.name]=x
 			i=skipWhitespace(s,i)
-			i,x=Property.create(s,i)
 		return (i,res)
 
 	def isGINode(self):
diff --git a/src/sgfParser/property.py b/src/sgfParser/property.py
--- a/src/sgfParser/property.py
+++ b/src/sgfParser/property.py
@@ -1,4 +1,5 @@
 import re
+from . import skipWhitespace, ParserError
 
 
 GAME_INFO=1
@@ -28,36 +29,54 @@ class Point:
 		return chr(a+self.c)+chr(a+self.r)
 
 
+## Metatype matching one of the provided types.
+#
+# Returns the first match, so the order is important.
 def choose(*vTypes):
 	def f(s,start):
 		for vType in vTypes:
-			i,x=vType(s,start)
-			if x is not None: return (i,x)
-		return (start,None)
+			try:
+				i,x=vType(s,start)
+				return (i,x)
+			except ParserError: pass
+		raise ParserError("no variant of a 'choose' property value matched",s,start)
 	return f
 
 
+def singletonFits(s,i):
+	return i<len(s) and s[i]=="["
+
+
+def singletonEnds(s,i):
+	return i<len(s) and s[i]=="]"
+
+
 def singleton(vType):
 	def f(s,start):
-		if s[start]!="[":
-			return (start,None)
+		if not singletonFits(s,start):
+			raise ParserError("expected a property value starting with '['",s,start)
 		i,x=vType(s,start+1)
-		if x is None: return (start,None)
-		if s[i]!="]":
-			return (start,None)
-		return (i+1,x)
+		if not singletonEnds(s,i):
+			raise ParserError("expected a property value ending with ']'",s,i)
+		i=skipWhitespace(s,i+1)
+		return (i,x)
 	return f
 
 
 def listOf(vType,allowEmpty=False):
 	def f(s,start):
-		res=[]
+		i=start
+		if not singletonFits(s,i):
+			raise ParserError("expected a property value starting with '['",s,i)
+		if singletonEnds(s,i+1) and allowEmpty:
+			i=skipWhitespace(s,i+2)
+			return (i,[])
 		single=singleton(vType)
-		i,x=single(s,start)
-		while x!=None:
+		i,x=single(s,i)
+		res=[x]
+		while singletonFits(s,i):
+			i,x=single(s,i)
 			res.append(x)
-			i,x=single(s,i)
-		if len(res)==0 and not allowEmpty: return (start,None)
 		return (i,res)
 	return f
 
@@ -65,9 +84,9 @@ def listOf(vType,allowEmpty=False):
 def compose(vTypeA,vTypeB):
 	def f(s,start):
 		i,a=vTypeA(s,start)
-		if a==None or s[i]!=":": return (start,None)
+		if i>=len(s) or s[i]!=":":
+			raise ParserError("a composed property value separated by ':' expected",s,i)
 		i,b=vTypeB(s,i+1)
-		if b==None: return start,None
 		return (i,Composed(a,b))
 	return f
 
@@ -75,7 +94,7 @@ def compose(vTypeA,vTypeB):
 def number(s,start):
 	r=re.compile(r"(\+|-|)\d+")
 	m=r.match(s,start)
-	if m is None: return (start,None)
+	if m is None: raise ParserError("expected a number matching '(\+|-|)\d+'",s,start)
 	res=int(m.group(0))
 	return (m.end(),res)
 
@@ -83,7 +102,7 @@ def number(s,start):
 def real(s,start):
 	r=re.compile(r"(\+|-|)\d+(\.\d+)?")
 	m=r.match(s,start)
-	if m is None: return (start,None)
+	if m is None: raise ParserError("expected a real number matching '(\+|-|)\d+(\.\d+)?'",s,start)
 	res=float(m.group(0))
 	return (m.end(),res)
 
@@ -91,7 +110,7 @@ def real(s,start):
 def double(s,start):
 	r=re.compile(r"1|2")
 	m=r.match(s,start)
-	if m is None: return (start,None)
+	if m is None: raise ParserError("expected a double value, either '1' or '2'",s,start)
 	res=int(m.group(0))
 	return (m.end(),res)
 
@@ -99,7 +118,7 @@ def double(s,start):
 def color(s,start):
 	r=re.compile(r"B|W")
 	m=r.match(s,start)
-	if m is None: return (start,None)
+	if m is None: raise ParserError("expected a color value, either 'B' or 'W'",s,start)
 	return (m.end(),m.group(0))
 
 
@@ -108,6 +127,7 @@ def text(simple=True,composed=False):
 		res=""
 		esc=False
 		lastC=""
+		i=start
 		for i,c in enumerate(s[start:],start):
 			if esc:
 				if c!="\n" and c!="\r": res+=c
@@ -144,7 +164,7 @@ def anything(s,start):
 def point(s,start):
 	r=re.compile(r"[a-zA-Z]{2}|") # !! limit to board size
 	m=r.match(s,start)
-	if m is None: return (start,None)
+	if m is None: raise ParserError("expected a point value matching '[a-zA-Z]{2}|'",s,start)
 	if m.group(0)=="": # pass, !! tt
 		return (m.end(),tuple())
 	col=m.group(0)[0]
@@ -158,29 +178,31 @@ stone=point
 
 
 class Property:
+	identRegexp=re.compile(r"[A-Z]+")
+
 	def __init__(self):
 		self.name=""
 		self.value=""
 
 	@staticmethod
+	def fits(s,i):
+		return i<len(s) and s[i].isupper()
+
+	@staticmethod
 	def create(s,start):
+		assert Property.fits(s,start)
 		res=Property()
-		i,x=Property.ident(s,start)
-		if x is None:
-			return (start,None)
-		res.name=x
+		i,res.name=Property.ident(s,start)
+		i=skipWhitespace(s,i)
 		i,x=Property.createValue(s,i,res.name)
-		if x is None:
-			print('error when parsing property "{0}" at position {1}'.format(res.name,i))
-			return (start,None)
 		res.value=x
+		i=skipWhitespace(s,i)
 		return (i,res)
 
 	@staticmethod
 	def ident(s,start):
-		r=re.compile(r"[A-Z]+")
-		m=r.match(s,start)
-		if m is None: return (start,None)
+		m=Property.identRegexp.match(s,start)
+		if m is None: raise ParserError("expected a property identifier matching '[A-Z]+'",s,start)
 		return (m.end(),m.group())
 
 	@staticmethod
@@ -188,8 +210,9 @@ class Property:
 		if name in Property.patterns:
 			return Property.patterns[name](s,start)
 		else:
-			print('warning, unknown property "{0}" at position {1}'.format(name,start))
-			return singleton(anything)(s,start)
+			# !! raise or log or ignore
+			# print('warning, unknown property "{0}" at position {1}'.format(name,start))
+			return choose(listOf(anything)(s,start), singleton(anything)(s,start))
 
 	@property
 	def type(self):
diff --git a/src/tests/testSgfParser.py b/src/tests/testSgfParser.py
--- a/src/tests/testSgfParser.py
+++ b/src/tests/testSgfParser.py
@@ -11,8 +11,10 @@ dataDir=os.path.join(os.path.dirname(__f
 
 class TestProperty(TestCase):
 	def testName(self):
-		self.assertEqual(Property.create("[99]",0), (0,None))
-		self.assertEqual(Property.create("99[99]",0), (0,None))
+		with self.assertRaises(AssertionError):
+			Property.create("[99]",0)
+		with self.assertRaises(AssertionError):
+			Property.create("99[99]",0)
 
 		i,prop=Property.create("MN[99]",0)
 		self.assertNotEqual((i,prop), (0,None))