diff --git a/boolean/boolean.py b/boolean/boolean.py
index 2c17473..10aa3d3 100644
--- a/boolean/boolean.py
+++ b/boolean/boolean.py
@@ -22,6 +22,7 @@
 
 from __future__ import absolute_import
 from __future__ import unicode_literals
+from __future__ import print_function
 
 import inspect
 import itertools
@@ -31,6 +32,8 @@
 except NameError:
     basestring = str  # Python 3
 
+# Set to True to enable tracing for parsing
+TRACE_PARSE = False
 
 # Token types for standard operators and parens
 TOKEN_AND = 1
@@ -59,12 +62,14 @@
 PARSE_UNBALANCED_CLOSING_PARENS = 2
 PARSE_INVALID_EXPRESSION = 3
 PARSE_INVALID_NESTING = 4
+PARSE_INVALID_SYMBOL_SEQUENCE = 5
 
 PARSE_ERRORS = {
     PARSE_UNKNOWN_TOKEN: 'Unknown token',
     PARSE_UNBALANCED_CLOSING_PARENS: 'Unbalanced parenthesis',
     PARSE_INVALID_EXPRESSION: 'Invalid expression',
-    PARSE_INVALID_NESTING: 'Invalid expression nesting such as (AND xx)'
+    PARSE_INVALID_NESTING: 'Invalid expression nesting such as (AND xx)',
+    PARSE_INVALID_SYMBOL_SEQUENCE: 'Invalid symbols sequence such as (A B)',
 }
 
 
@@ -197,27 +202,54 @@ def parse(self, expr, simplify=False):
         else:
             tokenized = iter(expr)
 
+        if TRACE_PARSE:
+            tokenized = list(tokenized)
+            print('tokens:')
+            map(print, tokenized)
+            tokenized = iter(tokenized)
+
         ast = [None, None]
 
+        def is_sym(_t):
+            return _t == TOKEN_SYMBOL or isinstance(_t, Symbol)
+
         prev = None
         for tok in tokenized:
+            if TRACE_PARSE: print('\nprocessing token:', repr(tok))
             token, tokstr, position = tok
+
+            if prev:
+                prev_token, _, _ = prev
+                if is_sym(prev_token) and is_sym(token):
+                    raise ParseError(token, tokstr, position, PARSE_INVALID_SYMBOL_SEQUENCE)
+
             if token == TOKEN_SYMBOL:
                 ast.append(self.Symbol(tokstr))
+                if TRACE_PARSE: print(' ast: token == TOKEN_SYMBOL: append new symbol', repr(ast))
+
             elif isinstance(token, Symbol):
                 ast.append(token)
+                if TRACE_PARSE: print(' ast: isinstance(token, Symbol): append existing symbol', repr(ast))
 
             elif token == TOKEN_TRUE:
                 ast.append(self.TRUE)
+                if TRACE_PARSE: print('ast4:', repr(ast))
+
             elif token == TOKEN_FALSE:
                 ast.append(self.FALSE)
+                if TRACE_PARSE: print('ast5:', repr(ast))
 
             elif token == TOKEN_NOT:
                 ast = [ast, self.NOT]
+                if TRACE_PARSE: print('ast6:', repr(ast))
+
             elif token == TOKEN_AND:
                 ast = self._start_operation(ast, self.AND, precedence)
+                if TRACE_PARSE: print(' ast: token == TOKEN_AND: start_operation', repr(ast))
+
             elif token == TOKEN_OR:
                 ast = self._start_operation(ast, self.OR, precedence)
+                if TRACE_PARSE: print(' ast: token == TOKEN_OR: start_operation', repr(ast))
 
             elif token == TOKEN_LPAR:
                 if prev:
@@ -227,13 +259,16 @@ def parse(self, expr, simplify=False):
                     if ptoktype not in (TOKEN_NOT, TOKEN_AND, TOKEN_OR, TOKEN_LPAR):
                         raise ParseError(token, tokstr, position, PARSE_INVALID_NESTING)
                 ast = [ast, TOKEN_LPAR]
+
             elif token == TOKEN_RPAR:
                 while True:
                     if ast[0] is None:
                         raise ParseError(token, tokstr, position, PARSE_UNBALANCED_CLOSING_PARENS)
                     if ast[1] is TOKEN_LPAR:
                         ast[0].append(ast[2])
+                        if TRACE_PARSE: print('ast9:', repr(ast))
                         ast = ast[0]
+                        if TRACE_PARSE: print('ast10:', repr(ast))
                         break
 
                     if isinstance(ast[1], int):
@@ -246,10 +281,12 @@ def parse(self, expr, simplify=False):
 
                     subex = ast[1](*ast[2:])
                     ast[0].append(subex)
+                    if TRACE_PARSE: print('ast11:', repr(ast))
                     ast = ast[0]
+                    if TRACE_PARSE: print('ast12:', repr(ast))
             else:
                 raise ParseError(token, tokstr, position, PARSE_UNKNOWN_TOKEN)
-            
+
             prev = tok
 
         try:
@@ -259,16 +296,21 @@ def parse(self, expr, simplify=False):
                         if len(ast) != 3:
                             raise ParseError(error_code=PARSE_INVALID_EXPRESSION)
                         parsed = ast[2]
+                        if TRACE_PARSE: print('parsed1:', repr(parsed))
                     else:
                         parsed = ast[1](*ast[2:])
+                        if TRACE_PARSE: print('parsed2:', repr(parsed))
                     break
                 else:
                     subex = ast[1](*ast[2:])
                     ast[0].append(subex)
+                    if TRACE_PARSE: print('ast13:', repr(ast))
                     ast = ast[0]
+                    if TRACE_PARSE: print('ast14:', repr(ast))
         except TypeError:
             raise ParseError(error_code=PARSE_INVALID_EXPRESSION)
 
+        if TRACE_PARSE: print('parsed3:', repr(parsed))
         if simplify:
             return parsed.simplify()
         return parsed
@@ -277,18 +319,24 @@ def _start_operation(self, ast, operation, precedence):
         """
         Returns an AST where all operations of lower precedence are finalized.
         """
+        if TRACE_PARSE: print('   start_operation: ast, operation, precedence', repr(ast), repr(operation), repr(precedence))
         op_prec = precedence[operation]
         while True:
             if ast[1] is None:  # [None, None, x]
+                if TRACE_PARSE: print('       start_op: ast[1] is None:', repr(ast))
                 ast[1] = operation
+                if TRACE_PARSE: print('       --> start_op: ast[1] is None:', repr(ast))
                 return ast
 
             prec = precedence[ast[1]]
             if prec > op_prec:  # op=&, [ast, |, x, y] -> [[ast, |, x], &, y]
+                if TRACE_PARSE: print('       start_op: prec > op_prec:', repr(ast))
                 ast = [ast, operation, ast.pop(-1)]
+                if TRACE_PARSE: print('       --> start_op: prec > op_prec:', repr(ast))
                 return ast
 
             if prec == op_prec:  # op=&, [ast, &, x] -> [ast, &, x]
+                if TRACE_PARSE: print('       start_op: prec == op_prec:', repr(ast))
                 return ast
 
             if not (inspect.isclass(ast[1]) and issubclass(ast[1], Function)):
@@ -296,12 +344,17 @@ def _start_operation(self, ast, operation, precedence):
                 raise ParseError(error_code=PARSE_INVALID_NESTING)
 
             if ast[0] is None:  # op=|, [None, &, x, y] -> [None, |, x&y]
+                if TRACE_PARSE: print('       start_op: ast[0] is None:', repr(ast))
                 subexp = ast[1](*ast[2:])
-                return [ast[0], operation, subexp]
+                new_ast = [ast[0], operation, subexp]
+                if TRACE_PARSE: print('       --> start_op: ast[0] is None:', repr(new_ast))
+                return new_ast
 
             else:  # op=|, [[ast, &, x], ~, y] -> [ast, &, x, ~y]
+                if TRACE_PARSE: print('       start_op: else:', repr(ast))
                 ast[0].append(ast[1](*ast[2:]))
                 ast = ast[0]
+                if TRACE_PARSE: print('       --> start_op: else:', repr(ast))
 
     def tokenize(self, expr):
         """
diff --git a/boolean/test_boolean.py b/boolean/test_boolean.py
index cb3ef3e..778db5b 100644
--- a/boolean/test_boolean.py
+++ b/boolean/test_boolean.py
@@ -7,16 +7,18 @@
 Released under revised BSD license.
 """
 
-from __future__ import absolute_import, unicode_literals
-
-import unittest
-from unittest.case import expectedFailure
+from __future__ import absolute_import
+from __future__ import unicode_literals
+from __future__ import print_function
+from boolean.boolean import PARSE_UNKNOWN_TOKEN
 
 try:
     basestring  # Python 2
 except NameError:
     basestring = str  # Python 3
 
+import unittest
+from unittest.case import expectedFailure
 
 from boolean import BooleanAlgebra
 from boolean import ParseError
@@ -29,6 +31,9 @@
 from boolean import TOKEN_SYMBOL
 from boolean import TOKEN_LPAR
 from boolean import TOKEN_RPAR
+from boolean.boolean import PARSE_INVALID_SYMBOL_SEQUENCE
+from boolean.boolean import PARSE_INVALID_EXPRESSION
+from boolean.boolean import PARSE_INVALID_NESTING
 
 
 class BooleanAlgebraTestCase(unittest.TestCase):
@@ -281,13 +286,43 @@ def test_parse_raise_ParseError(self):
         ]
 
         for expr in invalid_expressions:
-            print(expr)
             try:
                 algebra.parse(expr)
                 self.fail("Exception should be raised when parsing '%s'" % expr)
-            except ParseError:
-                pass
+            except ParseError as pe:
+                assert pe.error_code == PARSE_UNKNOWN_TOKEN
 
+    def test_parse_side_by_side_symbols_should_raise_exception_but_not(self):
+        algebra = BooleanAlgebra()
+        expr_str = 'a or b c'
+        try:
+            algebra.parse(expr_str)
+        except ParseError as pe:
+            assert pe.error_code == PARSE_INVALID_SYMBOL_SEQUENCE
+
+    def test_parse_side_by_side_symbols_should_raise_exception_but_not2(self):
+        algebra = BooleanAlgebra()
+        expr_str = '(a or b) c'
+        try:
+            algebra.parse(expr_str)
+        except ParseError as pe:
+            assert pe.error_code == PARSE_INVALID_EXPRESSION
+
+    def test_parse_side_by_side_symbols_raise_exception(self):
+        algebra = BooleanAlgebra()
+        expr_str = 'a b'
+        try:
+            algebra.parse(expr_str)
+        except ParseError as pe:
+            assert pe.error_code == PARSE_INVALID_SYMBOL_SEQUENCE
+
+    def test_parse_side_by_side_symbols_with_parens_raise_exception(self):
+        algebra = BooleanAlgebra()
+        expr_str = '(a) (b)'
+        try:
+            algebra.parse(expr_str)
+        except ParseError as pe:
+            assert pe.error_code == PARSE_INVALID_NESTING
 
 class BaseElementTestCase(unittest.TestCase):
 
@@ -783,8 +818,11 @@ def test_simplify_complex_expression_parsed_then_simplified(self):
 
     def test_parse_invalid_nested_and_should_raise_a_proper_exception(self):
         algebra = BooleanAlgebra()
-        test_expression_str = '''a (and b)'''
-        self.assertRaises(ParseError, algebra.parse, test_expression_str)
+        expr = '''a (and b)'''
+        try:
+            algebra.parse(expr)
+        except ParseError as pe:
+            assert pe.error_code == PARSE_INVALID_NESTING
 
     def test_subtract(self):
         parse = BooleanAlgebra().parse