What item is at the top of the stack?

$m=$ Stack$()$

$m.$push$(\text{'}x\text{'})$

$m.$push$(\text{'}y\text{'})$

$m.$pop$()$

$m.$push$(\text{'}z\text{'})$

$m.$peek$()$

m$.$push$(\text{'}$z$\text{'})$

m$.$peek$()$

$\text{'}y$

$\text{'}x\text{'}$

$\text{'}$z$\text{'}$

The stack is empty