func (s *stack) peek() string {
	if s.data == nil || len(s.data) == 0 {
		return "" // normally error, but this isn't production code
	}
	return s.data[len(s.data)-1]
}
// look at value at top of stack without pop
look at value at top of stack without pop
// look at value at top of stack without pop
func (s *stack) peek() string {
	if s.data == nil || len(s.data) == 0 {
		return "" 
	}
	return s.data[len(s.data)-1]
}
func ( s * stack ) peek ( ) string { if s . data == nil || len ( s . data ) == 0 { return " " } return s . data [ len ( s . data ) - 1 ] }
look at value at top of stack without pop
look at value at top of stack without pop
// normally error, but this isn't production code