Consider the following execution of a stack implementation for three threads p$,$ q$,$ r and on a single object $($stack type$).$

three threads p$,$ q$,$ r and on a single object $($of type stack$).$

The sequential specification of a stack is that the push$($x$)$ method a $$nadds x to the start of the stack.

and the pop$()$ : x method removes the first element x at the top of the stack $($maintains LIF O$).$