code: func (s *exprStack) popN(n int) []Expression {
  	if n > s.len() || n < 0 {
  		n = s.len()
  	}
  	idx := s.len() - n
  	exprs := s.stack[idx:]
  	s.stack = s.stack[:idx]
  	return exprs
  }
docs: // popN pops n expressions from the stack.
  // If n greater than stack length or n is negative, it pops all the expressions.