def run(self, mod):
        """Find all assert statements in *mod* and rewrite them."""
        if not mod.body:
            # Nothing to do.
            return
        # Insert some special imports at the top of the module but after any
        # docstrings and __future__ imports.
        aliases = [ast.alias(py.builtin.builtins.__name__, "@py_builtins"),
                   ast.alias("dessert.rewrite", "@dessert_ar")]
        expect_docstring = True
        pos = 0
        lineno = 0
        for item in mod.body:
            if (expect_docstring and isinstance(item, ast.Expr) and
                    isinstance(item.value, ast.Str)):
                doc = item.value.s
                if "PYTEST_DONT_REWRITE" in doc:
                    # The module has disabled assertion rewriting.
                    return
                lineno += len(doc) - 1
                expect_docstring = False
            elif (not isinstance(item, ast.ImportFrom) or item.level > 0 or
                  item.module != "__future__"):
                lineno = item.lineno
                break
            pos += 1
        imports = [ast.Import([alias], lineno=lineno, col_offset=0)
                   for alias in aliases]
        mod.body[pos:pos] = imports
        # Collect asserts.
        nodes = [mod]
        while nodes:
            node = nodes.pop()
            for name, field in ast.iter_fields(node):
                if isinstance(field, list):
                    new = []
                    for i, child in enumerate(field):
                        if isinstance(child, ast.Assert):
                            # Transform assert.
                            new.extend(self.visit(child))
                        else:
                            new.append(child)
                            if isinstance(child, ast.AST):
                                nodes.append(child)
                    setattr(node, name, new)
                elif (isinstance(field, ast.AST) and
                      # Don't recurse into expressions as they can't contain
                      # asserts.
                      not isinstance(field, ast.expr)):
                    nodes.append(field)
Find all assert statements in *mod* and rewrite them.