diff --git a/.travis.yml b/.travis.yml
index 591fab9..6d7cf96 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,10 +3,10 @@ matrix:
     include:
         - python: 2.7
           env: TOX_ENV=py27
-        - python: 3.3
-          env: TOX_ENV=py33
         - python: 3.4
           env: TOX_ENV=py34
+        - python: 3.5
+          env: TOX_ENV=py35
 install:
     - "pip install tox coveralls"
 script:
diff --git a/dice/client/window.py b/dice/client/window.py
index b9bda8d..b12c513 100644
--- a/dice/client/window.py
+++ b/dice/client/window.py
@@ -99,7 +99,7 @@ def _dispatch_events(self):
         elif ch == ord('l'):
             app.show_log = not app.show_log
         elif ch == ord('s'):
-            app.last_item.save('saved_item.txt')
+            app.last_item.save('./saved_item.txt')
         elif ch == ord('\t'):
             cur_idx = self.panels.index(self.active_panel)
             next_idx = (cur_idx + 1) % len(self.panels)
diff --git a/dice/core/constraint.py b/dice/core/constraint.py
index 7bfae08..29d9a10 100644
--- a/dice/core/constraint.py
+++ b/dice/core/constraint.py
@@ -4,6 +4,7 @@
 import random
 import re
 import yaml
+import shutil
 
 from . import trace
 
@@ -89,14 +90,27 @@ def constrain(self, item):
         self.item = item
         self.status = {c.name: 'untouched'
                        for c in self.constraints}
+        cst_temp = self.constraints[:]
+        path_temp = None
         while any(s == 'untouched' for s in self.status.values()):
-            for constraint in self.constraints:
-                if self._assumption_valid(constraint):
-                    result = constraint.apply(item)
+            while len(cst_temp) > 0:
+                if self._assumption_valid(cst_temp[0]):
+                    result = cst_temp[0].apply(item)
+                    if result == "success":
+                        if cst_temp[0].child is not None:
+                            path_temp = os.path.join(self.provider.path,
+                                                     'oracles',
+                                                     cst_temp[0].child)
+                            cst_temp += self._load_constraints(path_temp)
+
                 else:
                     result = 'skipped'
 
-                self.status[constraint.name] = result
+                self.status[cst_temp[0].name] = result
+                cst_temp.remove(cst_temp[0])
+            if path_temp is not None:
+                if os.path.isdir(path_temp):
+                    shutil.rmtree(path_temp)
 
 
 class Constraint(object):
@@ -106,7 +120,8 @@ class Constraint(object):
     path_prefix = 'DPATH'
 
     def __init__(self, name, provider,
-                 depends_on=None, require=None, oracle=None):
+                 depends_on=None, require=None, child=None, oracle=None,
+                 fail_ratio=0.1, alpha=20, beta=1.8):
         """
         :param name: Unique string name of the constraint.
         :param depends_on: A logical expression shows prerequisite to apply
@@ -118,8 +133,11 @@ def __init__(self, name, provider,
         self.provider = provider
         self.depends_on = depends_on
         self.require = require
+        self.child = child
         self.oracle = oracle
-        self.fail_ratio = 0.1
+        self.fail_ratio = fail_ratio
+        self.alpha = alpha
+        self.beta = beta
         self.traces = self._oracle2traces(oracle)
 
     @classmethod
@@ -202,6 +220,9 @@ def _parse_if(node):
         def _parse_assert(node):
             cur_trace.append(node.test)
 
+        def _parse_expr(node):
+            cur_trace.append(node.value)
+
         def _parse_block(nodes):
             for node in nodes:
                 if isinstance(node, ast.If):
@@ -212,6 +233,8 @@ def _parse_block(nodes):
                     cur_trace.append(node)
                     traces.append(trace.Trace(self.provider, cur_trace))
                     cur_trace.pop()
+                elif isinstance(node, ast.Expr):
+                    _parse_expr(node)
                 else:
                     raise ConstraintError(
                         'Unknown node type: %s' % node.__class__.__name__)
@@ -290,7 +313,7 @@ def _name2path(name):
             return name[len(self.path_prefix):].replace('_', '/')
 
         t = self._choose()
-        sols = t.solve(item)
+        sols = t.solve(item, self.alpha, self.beta)
         for name, sol in sols.items():
             item.set(_name2path(name), sol)
 
diff --git a/dice/core/item.py b/dice/core/item.py
index 2509de4..54d25e3 100644
--- a/dice/core/item.py
+++ b/dice/core/item.py
@@ -38,3 +38,6 @@ def get(self, path):
         :return: Option value got.
         """
         return getattr(self, path, None)
+
+    def save(self, path="./saved_item.txt"):
+        pass  # TODO
diff --git a/dice/core/symbol.py b/dice/core/symbol.py
index 53585ac..aab97de 100644
--- a/dice/core/symbol.py
+++ b/dice/core/symbol.py
@@ -19,7 +19,7 @@ def __init__(self, scope=None, excs=None, exc_types=None):
         self.excs = excs
         self.exc_types = exc_types
 
-    def generate(self):
+    def generate(self, alpha=20, beta=1.8):
         """
         Generate a random instance of this symbol without considering scope,
         excs or exc_types. Must be overridden.
@@ -27,7 +27,7 @@ def generate(self):
         raise NotImplementedError("Method 'generate' not implemented for %s" %
                                   self.__class__.__name__)
 
-    def model(self):
+    def model(self, alpha=20, beta=1.8):
         """
         Generate a random instance of this symbol.
         """
@@ -35,7 +35,7 @@ def model(self):
             res = self.generate()
             if self.excs is not None:
                 while res in self.excs:
-                    res = self.generate()
+                    res = self.generate(alpha, beta)
             return res
         else:
             res = random.choice(self.scope)
@@ -49,11 +49,11 @@ class Bytes(SymbolBase):
     """
     Symbol class for a string contains random bytes (1~255).
     """
-    def generate(self):
+    def generate(self, alpha=20, beta=1.8):
         """
         Generate a random bytes string.
         """
-        cnt = int(random.weibullvariate(65535, 1))
+        cnt = int(random.weibullvariate(alpha, beta))
         return ''.join(bt for bt in os.urandom(cnt) if bt != b'\x00')
 
 
@@ -61,11 +61,11 @@ class NonEmptyBytes(Bytes):
     """
     Symbol class for a random byte(1-255) string except empty string.
     """
-    def generate(self):
+    def generate(self, alpha=20, beta=1.8):
         """
         Generate a random non-empty bytes string.
         """
-        cnt = int(random.weibullvariate(65535, 1)) + 1
+        cnt = int(random.weibullvariate(alpha, beta)) + 1
         return ''.join(bt for bt in os.urandom(cnt) if bt != b'\x00')
 
 
@@ -73,11 +73,11 @@ class String(Bytes):
     """
     Symbol class for a random printable string.
     """
-    def generate(self):
+    def generate(self, alpha=20, beta=1.8):
         """
         Generate a random printable string.
         """
-        cnt = int(random.weibullvariate(20, 1.8))
+        cnt = int(random.weibullvariate(alpha, beta))
         return ''.join(random.choice(string.printable) for _ in range(cnt))
 
 
@@ -94,18 +94,18 @@ def __init__(self, scope=None, excs=None, exc_types=None):
         super(StringList, self).__init__()
         self.scopes = []
 
-    def generate(self):
+    def generate(self, alpha=20, beta=1.8):
         """
         Generate a random printable strings.
         """
-        cnt = int(random.weibullvariate(20, 1.8))
+        cnt = int(random.weibullvariate(alpha, beta))
         return ''.join(random.choice(string.printable) for _ in range(cnt))
 
-    def model(self):
+    def model(self, alpha=3, beta=1.8):
         """
         Generate a random-numbered list contains random printable strings.
         """
-        cnt = int(random.weibullvariate(20, 1.8))
+        cnt = int(random.weibullvariate(alpha, beta))
         res = set()
         for _ in range(cnt):
             entry = None
@@ -146,16 +146,15 @@ def __repr__(self):
             minimum = '-Inf'
         return '<%s %s~%s>' % (self.__class__.__name__, minimum, maximum)
 
-    def generate(self):
+    def generate(self, alpha=30, beta=1.1):
         """
         Generate a random integer.
         """
-        scale = 50.0
         maximum = self.maximum
         minimum = self.minimum
         while True:
             sign = 1.0 if random.random() > 0.5 else -1.0
-            res = sign * (2.0 ** (random.expovariate(1.0 / scale)) - 1.0)
+            res = sign * (2.0 ** (random.weibullvariate(alpha, beta)) - 1.0)
             if maximum is not None:
                 if maximum >= 0 and res > maximum + 1:
                     continue
diff --git a/dice/core/trace.py b/dice/core/trace.py
index 30733ac..8a5df14 100644
--- a/dice/core/trace.py
+++ b/dice/core/trace.py
@@ -35,7 +35,6 @@ def __init__(self, provider, trace_list):
         assert isinstance(ret, ast.Return)
         self.result = ret.value.func.id.lower()
         args = ret.value.args
-
         self.result_patts = None
         if args:
             self.result_patts = args[0].s
@@ -55,7 +54,7 @@ def __repr__(self):
     def _exec_call(self, node):
         func_name = node.func.attr
         pkg_name = node.func.value.id
-        mod_name = '.'.join([self.provider.name, 'utils', pkg_name])
+        mod_name = '.'.join([self.provider.name + '_utils', pkg_name])
         mod = sys.modules[mod_name]
         func = getattr(mod, func_name)
         args = []
@@ -160,7 +159,10 @@ def _proc_compare(self, node):
 
     def _proc_call(self, node):
         func_name = node.func.id
-        assert func_name in ['any', 'all']
+        assert func_name in ['any', 'all', 'build']
+        if func_name == 'build':
+            self._exec_call(node.args[0])
+            return
         assert isinstance(node.args[0], ast.Compare)
         comp = node.args[0]
         op = comp.ops[0].__class__.__name__
@@ -189,7 +191,7 @@ def _proc_call(self, node):
             left = self._exec_call(left)
             if func_name == 'all':
                 if op == 'In':
-                    raise Exception('TODO')
+                    pass  # TODO
                 elif op == 'NotIn':
                     sym_right.excludes = left
             elif func_name == 'any':
@@ -198,10 +200,12 @@ def _proc_call(self, node):
         else:
             raise TraceError('Unknown left type %s' % left)
 
-    def solve(self, item):
+    def solve(self, item, alpha=20, beta=1.8):
         """
         Generate a satisfiable random option according to this trace.
         :param item: Item to which generated option applies.
+        :param alpha: Alpha to Weibull distribution.
+        :param beta: Beta to Weibull distribution.
         :return: Generated random option.
         """
         self.item = item
@@ -215,7 +219,7 @@ def solve(self, item):
             elif isinstance(node, ast.Return):
                 result = {}
                 for name, sym in self.symbols.items():
-                    result[name] = sym.model()
+                    result[name] = sym.model(alpha, beta)
                 return result
             else:
                 raise TraceError('Unknown node type: %s' % type(node))
diff --git a/tox.ini b/tox.ini
index 3f0966f..7ce909d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27, py33, py34, pep8, pylint, docs
+envlist = py27, py34, py35, pep8, pylint, docs
 
 [testenv]
 commands =