code: def assignment(self, node, children):
          'assignment = lvalue "=" expr'
          lvalue, _, expr = children
          self.env[lvalue] = expr
          return expf assignment(self, node, children):
          'assignment = lvalue "=" expr'
          lvalue, _, expr = children
          self.env[lvalue] = expr
          return expr
docs: assignment = lvalue "=" expr