diff --git a/docs/arithmetics.rst b/docs/arithmetics.rst
index cfdd31985eadf..fffe956f3ec17 100644
--- a/docs/arithmetics.rst
+++ b/docs/arithmetics.rst
@@ -26,7 +26,7 @@ Arithmetic operators
     print(2 % 3)   # 2
     print(-2 % 3)  # 1
 
-  For C-style mod, please use ``ti.raw_mod``:
+  For C style mod, please use ``ti.raw_mod``:
 
   .. code-block:: python
 
diff --git a/docs/vector.rst b/docs/vector.rst
index 17ddb844f8daf..d4881b98ac285 100644
--- a/docs/vector.rst
+++ b/docs/vector.rst
@@ -215,7 +215,10 @@ Methods
         a = ti.Vector([1.6, 2.3])
         a.cast(ti.i32) # [2, 3]
 
+    See :ref:`type` for more details.
+
 .. note::
+
     Vectors are special matrices with only 1 column. In fact, ``ti.Vector`` is just an alias of ``ti.Matrix``.
 
 
@@ -240,4 +243,6 @@ Metadata
         a = ti.Vector.field(3, dtype=ti.f32, shape=())
         a.n  # 3
 
+    See :ref:`meta` for more details.
+
 TODO: add element wise operations docs
diff --git a/python/taichi/lang/common_ops.py b/python/taichi/lang/common_ops.py
index f5e5eb066630b..909eec1d5af75 100644
--- a/python/taichi/lang/common_ops.py
+++ b/python/taichi/lang/common_ops.py
@@ -4,11 +4,6 @@ def __neg__(self):
         _taichi_skip_traceback = 1
         return ti.neg(self)
 
-    def __abs__(self):
-        import taichi as ti
-        _taichi_skip_traceback = 1
-        return ti.abs(self)
-
     def __add__(self, other):
         import taichi as ti
         _taichi_skip_traceback = 1
diff --git a/python/taichi/lang/impl.py b/python/taichi/lang/impl.py
index 9ffe22702526a..e3d0a034976f8 100644
--- a/python/taichi/lang/impl.py
+++ b/python/taichi/lang/impl.py
@@ -72,6 +72,17 @@ def begin_frontend_struct_for(group, loop_range):
     taichi_lang_core.begin_frontend_struct_for(group, loop_range.ptr)
 
 
+def begin_frontend_if(cond):
+    if is_taichi_class(cond):
+        raise ValueError(
+            'The truth value of vectors / matrices is ambiguous.\n'
+            'Consider using `any` or `all` when comparing vectors:\n'
+            '    if all(x == y):\n'
+            'or\n'
+            '    if any(x != y):\n')
+    taichi_lang_core.begin_frontend_if(Expr(cond).ptr)
+
+
 def wrap_scalar(x):
     if type(x) in [int, float]:
         return Expr(x)
diff --git a/python/taichi/lang/transformer.py b/python/taichi/lang/transformer.py
index 8db97e0f0eb7f..12b545bbc9656 100644
--- a/python/taichi/lang/transformer.py
+++ b/python/taichi/lang/transformer.py
@@ -324,7 +324,7 @@ def visit_If(self, node):
         template = '''
 if 1:
   __cond = 0
-  ti.core.begin_frontend_if(ti.Expr(__cond).ptr)
+  ti.begin_frontend_if(__cond)
   ti.core.begin_frontend_if_true()
   ti.core.pop_scope()
   ti.core.begin_frontend_if_false()