push = Function[{queue, priority, item},
   queue = SortBy[Append[queue, {priority, item}], First], HoldFirst];
pop = Function[queue,
   If[Length@queue == 0, Null,
    With[{item = queue[[-1, 2]]}, queue = Most@queue; item]],
   HoldFirst];
peek = Function[queue,
   If[Length@queue == 0, Null, Max[queue[[All, 1]]]], HoldFirst];
merge = Function[{queue1, queue2},
   SortBy[Join[queue1, queue2], First], HoldAll];