code: def visit_BinOp(self, node):
          
          res = combine(node.op, self.visit(node.left), self.visit(node.right))
          return self.add(node, res)
docs: Combine operands ranges for given operator.
  
          >>> import gast as ast
          >>> from pythran import passmanager, backend
          >>> node = ast.parse('''
          ... def foo():
          ...     a = 2
          ...     c = 3
          ...     d = a - c''')
          >>> pm = passmanager.PassManager("test")
          >>> res = pm.gather(RangeValues, node)
          >>> res['d']
          Interval(low=-1, high=-1)