POP_ASSUM_LIST : (thm list -> tactic) -> tactic
Generates a tactic from the assumptions, discards the assumptions and applies the tactic.
When applied to a function and a goal, POP_ASSUM_LIST applies the function to a list of theorems corresponding to the ASSUMEd assumptions of the goal, then applies the resulting tactic to the goal with an empty assumption list.
    POP_ASSUM_LIST f ({A1,...,An} ?- t) = f [A1 |- A1, ..., An |- An] (?- t)

Fails if the function fails when applied to the list of ASSUMEd assumptions, or if the resulting tactic fails when applied to the goal with no assumptions.
There is nothing magical about POP_ASSUM_LIST: the same effect can be achieved by using ASSUME a explicitly wherever the assumption a is used. If POP_ASSUM_LIST is used, it is unwise to select elements by number from the ASSUMEd-assumption list, since this introduces a dependency on ordering.
Suppose we have a goal of the following form:
   {a /\ b, c, (d /\ e) /\ f} ?- t
Then we can split the conjunctions in the assumption list apart by applying the tactic:
which results in the new goal:
   {a, b, c, d, e, f} ?- t

Making more delicate use of the assumption list than simply rewriting or using resolution.
HOL  Kananaskis-13