Parse an if construct.

def parse_if(self):
        """Parse an if construct."""
        node = result = nodes.If(lineno=self.stream.expect('name:if').lineno)
        while 1:
            node.test = self.parse_tuple(with_condexpr=False)
            node.body = self.parse_statements(('name:elif', 'name:else',
                                               'name:endif'))
            node.elif_ = []
            node.else_ = []
            token = next(self.stream)
            if token.test('name:elif'):
                node = nodes.If(lineno=self.stream.current.lineno)
                result.elif_.append(node)
                continue
            elif token.test('name:else'):
                result.else_ = self.parse_statements(('name:endif',),
                                                     drop_needle=True)
            break
        return result