diff --git a/lib/cretonne/meta/check.sh b/lib/cretonne/meta/check.sh
index e1e8b1f0b2..b0a6c853db 100755
--- a/lib/cretonne/meta/check.sh
+++ b/lib/cretonne/meta/check.sh
@@ -2,9 +2,6 @@
 set -e
 cd $(dirname "$0")
 
-echo "=== Python unit tests ==="
-python -m unittest discover
-
 runif() {
     if command -v "$1" > /dev/null; then
         echo "=== $1 ==="
@@ -19,13 +16,17 @@ runif() {
 # Install pylint with 'pip install pylint'.
 runif pylint --py3k --reports=no -- *.py cretonne isa
 
-# Then run the unit tests again with Python 3.
-# We get deprecation warnings about assertRaisesRegexp which was renamed in
-# Python 3, but there doesn't seem to be an easy workaround.
-runif python3 -Wignore:Deprecation -m unittest discover
-
 # Style linting.
 runif flake8 .
 
 # Type checking.
 runif mypy --py2 build.py
+
+echo "=== Python unit tests ==="
+python -m unittest discover
+
+# Then run the unit tests again with Python 3.
+# We get deprecation warnings about assertRaisesRegexp which was renamed in
+# Python 3, but there doesn't seem to be an easy workaround.
+runif python3 -Wignore:Deprecation -m unittest discover
+
diff --git a/lib/cretonne/meta/cretonne/ast.py b/lib/cretonne/meta/cretonne/ast.py
index 693a0b5a2b..a43ddfadcb 100644
--- a/lib/cretonne/meta/cretonne/ast.py
+++ b/lib/cretonne/meta/cretonne/ast.py
@@ -73,6 +73,24 @@ class Expr(object):
 class Var(Expr):
     """
     A free variable.
+
+    When variables are used in `XForms` with source ans destination patterns,
+    they are classified as follows:
+
+    Input values
+        Uses in the source pattern with no preceding def. These may appear as
+        inputs in the destination pattern too, but no new inputs can be
+        introduced.
+    Output values
+        Variables that are defined in both the source and destination pattern.
+        These values may have uses outside the source pattern, and the
+        destination pattern must compute the same value.
+    Intermediate values
+        Values that are defined in the source pattern, but not in the
+        destination pattern. These may have uses outside the source pattern, so
+        the defining instruction can't be deleted immediately.
+    Temporary values
+        Values that are defined only in the destination pattern.
     """
 
     def __init__(self, name):
@@ -82,15 +100,42 @@ class Var(Expr):
         # See XForm._rewrite_defs().
         self.defctx = 0
 
+    # Context bits for `defctx` indicating which pattern has defines of this
+    # var.
+    SRCCTX = 1
+    DSTCTX = 2
+
     def __str__(self):
+        # type: () -> str
         return self.name
 
     def __repr__(self):
+        # type: () -> str
         s = self.name
         if self.defctx:
             s += ", d={:02b}".format(self.defctx)
         return "Var({})".format(s)
 
+    def is_input(self):
+        # type: () -> bool
+        """Is this an input value to the source pattern?"""
+        return self.defctx == 0
+
+    def is_output(self):
+        """Is this an output value, defined in both src and dest patterns?"""
+        # type: () -> bool
+        return self.defctx == self.SRCCTX | self.DSTCTX
+
+    def is_intermediate(self):
+        """Is this an intermediate value, defined only in the src pattern?"""
+        # type: () -> bool
+        return self.defctx == self.SRCCTX
+
+    def is_temp(self):
+        """Is this a temp value, defined only in the dest pattern?"""
+        # type: () -> bool
+        return self.defctx == self.DSTCTX
+
 
 class Apply(Expr):
     """
diff --git a/lib/cretonne/meta/cretonne/legalize.py b/lib/cretonne/meta/cretonne/legalize.py
index 21073d0aec..8dfa2e6b97 100644
--- a/lib/cretonne/meta/cretonne/legalize.py
+++ b/lib/cretonne/meta/cretonne/legalize.py
@@ -114,5 +114,5 @@ expand.legalize(
         Rtl(
             (a1, b1) << isub_bout(x, y),
             (a, b2) << isub_bout(a1, b_in),
-            c << bor(b1, b2)
+            b << bor(b1, b2)
         ))
diff --git a/lib/cretonne/meta/cretonne/xform.py b/lib/cretonne/meta/cretonne/xform.py
index 5149c02991..2888b9c26c 100644
--- a/lib/cretonne/meta/cretonne/xform.py
+++ b/lib/cretonne/meta/cretonne/xform.py
@@ -11,10 +11,6 @@ except ImportError:
     pass
 
 
-SRCCTX = 1
-DSTCTX = 2
-
-
 def canonicalize_defapply(node):
     # type: (DefApply) -> Def
     """
@@ -87,13 +83,13 @@ class XForm(object):
         # Rewrite variables in src and dst RTL lists to our own copies.
         # Map name -> private Var.
         symtab = dict()  # type: Dict[str, Var]
-        self._rewrite_rtl(src, symtab, SRCCTX)
+        self._rewrite_rtl(src, symtab, Var.SRCCTX)
         num_src_inputs = len(self.inputs)
-        self._rewrite_rtl(dst, symtab, DSTCTX)
+        self._rewrite_rtl(dst, symtab, Var.DSTCTX)
 
         # Check for inconsistently used inputs.
         for i in self.inputs:
-            if i.defctx:
+            if not i.is_input():
                 raise AssertionError(
                         "'{}' used as both input and def".format(i))
 
@@ -189,6 +185,22 @@ class XForm(object):
                 self.inputs.append(var)
             yield var
 
+    def verify_legalize(self):
+        # type: () -> None
+        """
+        Verify that this is a valid legalization XForm.
+
+        - The source pattern must describe a single instruction.
+        - All values defined in the output pattern must be defined in the
+          destination pattern.
+        """
+        assert len(self.src.rtl) == 1, "Legalize needs single instruction."
+        defs, expr = self.src.rtl[0].defs_expr()
+        for d in defs:
+            if not d.is_output():
+                raise AssertionError(
+                        '{} not defined in dest pattern'.format(d))
+
 
 class XFormGroup(object):
     """
@@ -209,4 +221,6 @@ class XFormGroup(object):
         :param src: Single `Def` or `Apply` to be legalized.
         :param dst: `Rtl` list of replacement instructions.
         """
-        self.xforms.append(XForm(Rtl(src), dst))
+        xform = XForm(Rtl(src), dst)
+        xform.verify_legalize()
+        self.xforms.append(xform)
diff --git a/lib/cretonne/meta/gen_legalizer.py b/lib/cretonne/meta/gen_legalizer.py
index 0fdfc921af..2a2387723f 100644
--- a/lib/cretonne/meta/gen_legalizer.py
+++ b/lib/cretonne/meta/gen_legalizer.py
@@ -14,7 +14,7 @@ from cretonne.ast import Def, Apply  # noqa
 from cretonne.xform import XForm, XFormGroup  # noqa
 
 try:
-    from typing import Union  # noqa
+    from typing import Union
     DefApply = Union[Def, Apply]
 except ImportError:
     pass